a method for processing speech recognition including a system for generating a first probability of the word or sequence that words in a speech utterance is rejectable and a second probability that the word or sequence of words in the utterance is acceptable . a new selected phoneme rejection grammar is constructed of phonemes and / or merged phonemes . the highest first probability is calculated by comparing the utterance to sequence of the rejection grammar phonemes and / or merged phonemes . a main grammar is implemented in which the highest second probability is calculated that the utterance is composed of words in the main grammar . the two highest probabilities are compared and the utterance is accepted or rejected depending on the higher of the two highest probabilities . if accepted a sensible response may be made as determined by the specific application . if it is more probable that the sequence is in the rejection grammar , the utterance is rejected and as appropriate response as determined by the specific application is returned . the rejection grammar constructed by the new method is of small size , comprising often thirty or fewer selected phonemes . however , even with the small sized rejection grammar it is practical for use with small and large main grammars .