So obviously there's overlap between kanji and hanzi...so if you learn all the jouyou kanji you can use most of that knowledge in learning the hanzi necessary for Mandarin.

But what about the compounds of two or more kanji? Is there much of a correspondence between the kanji compounds in Japanese and the compounds in Mandarin?

Most compounds with onyomi are imported vocabulary from Chinese (not necessarily Mandarin), so most of them will exist and will often mean the same thing in Chinese. However, since quite some time has passed since they were imported, quite a few compounds have either fallen into disuse or have developed a slightly different meaning. There is also the issue of 和製漢語, which will only exist in Japanese unless it in turn has been imported into the Chinese.

So yes, as long as you're aware of the nuance differences, the correspondence is definitely big enough to give you a good head start when learning Mandarin

Edit: I should also note that while the meanings may be the same, the frequency of usage of the compound could be completely different. So you should by no means simply transfer your vocab from Japanese into Mandarin.

It's about the same level of overlap with compounds as with kanji-hanzi. Just as a random sample, here's the first sentence from today's featured article on Chinese Wikipedia (about the 77th Oscars):
第77届奥斯卡颁奖典礼是美国电影艺术与科学学院旨在奖励2004年最优秀电影的一场晚会，于太平洋时区2005年2月27日下午17点30分（北美东部时区晚上20点30分）开始在美国加利福尼亚州洛杉矶好莱坞的杜比剧院举行，共计颁发了24个类别的奥斯卡金像奖（也称学院奖）。

美国电影艺术与科学学院 is the Chinese name of the Academy of Arts and Sciences. The US is 美国 rather than 米国, movie is 电影（電影） rather than 映画, but then 艺术 (芸術) and 科学 are the same. アカデミー is used instead of 学院 here but the meaning is apparent.

Other words here: 时区（時区) is "time zone" (時間帯 in Japanese), 下午 is "afternoon", 晚上 is "evening", 17点30分 is 5:30 (rather than 17時30分), 开始(開始) is "start" (as in Japanese), 类别（類別) is "separated by category" (as in Japanese).

The Western names, of course, are nonsensical in Japanese (奥斯卡 for "Oscar", 加利福尼亚 for "California", etc.)

And of course the Japanese doesn't help you with the pronunciation at all.

Even the compounds that aren't used in Japanese, the meaning is often apparent if you have a good handle on Japanese.

yudantaiteki Wrote:And of course the Japanese doesn't help you with the pronunciation at all.

While you won't be able to take immediate use out of your knowledge in the pronunciation in Japanese, you can still make use of your knowledge to learn the correct pronunciations faster than someone without any such knowledge. While the pronunciations may seem completely different, they both follow the same system.

Take 章彰障漳璋蟑 (some of which are rare in Japanese) for instance which are all しょう in Japanese and all zhang in Mandarin (excluding tones). Or 譲壌攘穣 which are all じょう in Japanese and rang in Mandarin (again, excluding tones).

It probably won't do you too much good in the beginning, but I have personally found that with some experience, I can guess the reading of a kanji based of my knowledge in Japanese (and sometimes the other way around). For example, I have found the the Japanese reading ひつ usually maps to the Mandarin reading "bi" (必筆畢弼 etc).

Anyway my point is that your knowledge of Japanese readings isn't completely useless.

I went to Taiwan recently and I was able to understand a lot of signs and such, even though they use traditional Chinese. I remember seeing things like 故障, 出車注意 and 杏仁豆腐. Sometimes the readings are the "same", e.g. a station named 三重 (Sanchou), or the "an" in 保安 (Bao-an).

I think you'd have a big advantage over somebody starting from scratch. I imagine the differences in writing would be a bit of a pain though. I know 気 has been simplified from 氣, for example, and many Chinese simplified characters can look quite different from the equivalent Japanese versions. I seem to remember being told that the stroke order, e.g. for 王, can be different too.