This study was concerned with the nature of the character-comparison process involved in same-different judgments of arrays of letters. One condition replicated the standard matching task, with subjects pressing one key to indicate that the two arrays of letters were identical in all positions, and another key to indicate difference in one or more positions. A second condition reversed the standard decision rule and required the subjects to distinguish between arrays that differed in every position from those that matched in one or more positions. Reaction times were shorter to identical arrays in both conditions, indicating that identity effects observed in this and previous studies must be attributed to identity per se and not to decision or response factors. Mathematical analysis of the data revealed that, although a serial self-terminating model could be modified to give a good description of processing in this task, a model based on parallel processing was able to provide a better account both quantitatively and qualitatively. The analysis also indicated that subjects reduced their average reaction times by terminating the comparison process prematurely on some trials, and this guessing strategy was shown to have important implications for the shapes of the reaction time functions observed in this task.