A method and system for testing voice applications, such as VoiceXML applications, is provided. The system provides a run-time simulation environment for voice applications that simulates and automates user interaction. A user simulation script is provided in a customized mark-up language. The voice application is processed to derive a nominal output of the voice application. The user simulation script is processed to generate a simulated output for the voice application corresponding to the nominal output. Conditional logic may be applied to the nominal output to generate a simulated input in response thereto. The user simulation script is specified in a customized mark-up language having a set of one or more conditional tags and an internal variable for the nominal output of the voice application.