java对象序列化是否在1.5和1.6之间兼容

I am wondering whether it is safe to mix jdk 1.5 and 1.6 (Java 6) object serialization (biderctional communication). I searched for an explicit statement from sun concerning this question but did not succeed. So, besides the technical feasability I am searching for an "official" statement concerning the problem.I am wondering whether it is safe to mix jdk 1.