One aspect of autistic language that has been infrequently researched is vocabulary and the conceptual knowledge underpinning individual words or word types. In this descriptive study we investigate anomalous vocabulary use in a 70,000-word corpus of conversational autistic language and examine evidence that concept formation, and hence vocabulary, is abnormal in autism. Particular attention is paid to the expression of artifact and temporal concepts which some believe may develop abnormally in autism. Little evidence is found of anomalous use of artifact terms, though errors with temporal (and also spatial) expressions are relatively common. We discuss why this may be and consider several potential explanations for why underlying lexical knowledge in autism may not necessarily be reflected in lexical use.