Code ======== ABC='ABC' puts "ABC[0] encoding is #{ABC[0].encoding}" puts "?\\xff encoding is #{?\xff.encoding}" Output ======== MRI output ABC[0] encoding is US-ASCII ?\xff encoding is ASCII-8BIT macruby output ABC[0] encoding is UTF-8 ?\xff encoding is UTF-8 The encodings reported above did not seem to be effected by the encoding of the source file. I tried both ASCII and UTF-8. When the same code is executed in (mac)irb the results are the same for macirb as they are for macruby. irb for MRI however produces UTF-8 strings in both cases! This seemed very odd but I'm fairly sure it's because I have an environment variable: LANG=GB.UTF-8 When I changed to LANG=GB.US_ASCII irb for MRI rendered 'abc'[0] with US_ASCII encoding. macirb still used UTF-8. (I discovered this when trying to get ruby-mysql to work with macruby. It doesn't work as-is but seems to work with a few mods that use force_encoding to make MRI and macruby produce the same outputs). I abandoned my earlier attempts to use postgres with macruby via the pg gem. It failed regularly but in unpredictable ways associated, as far as I could tell, with memory management problems.