I went back to this today to take it a step further and found my way blocked - it seems to be a MacRuby thing because this is trivial to implement in Objective-C. This code is now: framework 'Cocoa' class AppDelegate def initialize registerExperimental @eventDescriptor = NSAppleEventDescriptor.descriptorWithString("this_is_a_string") end def registerExperimental sharedAEManager = NSAppleEventManager.sharedAppleEventManager sharedAEManager.setEventHandler(self, andSelector: :"getURL:event:withReplyEvent:returnEvent", forEventClass: KInternetEventClass, andEventID:KAEGetURL) end def getURL(event, withReplyEvent:returnEvent) if event.respond_to?(:paramDescriptorForKeyword) url = event.paramDescriptorForKeyword(keyDirectObject).to_s end end def applicationShouldTerminateAfterLastWindowClosed(theApplication) return true; end end I've edited the info.plist to register 'experimental' as a custom URL scheme and tested it by calling it in Safari. Typing 'experimental://' in the Safari address bar produces an error: experimental[4759]: -[AppDelegate getURL:event:withReplyEvent:returnEvent]: unrecognised selector sent to instance 0x200c0e240 I've tried re-writing the 'andSelector: :"getURL:event:withReplyEvent:returnEvent"' part of the setEventHandler call every way I can think of: andSelector:"getURL:event:withReplyEvent:returnEvent" andSelector:"getURL:withReplyEvent" to name but two. I'm fairly sure the whole method name has to be sent, including the argument names, so I can't really see what is wrong. As I say, I've constructed the Obj-C equivalent (experimental-cpp) where typing 'experimental-cpp:///splat' produces exactly the required response. Any ideas anyone, please?