The VT page currently says this program works with 10.0 thru 10.3.9, but that seems unlikely. According to the download page, Phex 3.0 requires "Java 2 Standard Edition (J2SE) 1.5," also known as J2SE 5.0. At present, Apple has only released that version of Java for 10.4 Tiger. See: http://developer.apple.com/technotes/tn2002/tn2110.html#TABLES. Therefore Phex probably can't run on pre-Tiger systems.