The influence of listener's expertise and sound identification on the categorization of environmental sounds is reported in three studies. In Study 1, the causal uncertainty of 96 sounds was measured by counting the different causes described by 29 participants. In Study 2, 15 experts and 15 nonexperts classified a selection of 60 sounds and indicated the similarities they used. In Study 3, 38 participants indicated their confidence in identifying the sounds. Participants reported using either acoustical similarities or similarities of the causes of the sounds. Experts used acoustical similarity more often than nonexperts, who used the similarity of the cause of the sounds. Sounds with a low causal uncertainty were more often grouped together because of the similarities of the cause, whereas sounds with a high causal uncertainty were grouped together more often because of the acoustical similarities. The same conclusions were reached for identification confidence. This measure allowed the sound classification to be predicted, and is a straightforward method to determine the appropriate description of a sound. (Contains 4 tables and 6 figures.)