Similarity comparisons are highly sensitive to judgment context. Three experiments explore context effects that occur within a single comparison rather than across several trials. Experiment 1 shows reliable intransitivities in which a target is judged to be more similar to stimulus A than to stimulus B, more similar to B than to stimulus C, and more similar to C than to A. Experiment 2 explores the locus of Tversky`s (1977) diagnosticity effect in which the relative similarity of two alternatives to a target is influenced by a third alternative. Experiment 3 demonstrates reliable, though occasional, violations of an assumption of monotonicity. The observed violations of common assumptions to many models of similarity can be accomodated in terms of dynamic property weighting processes based on specific forms of diagnosticity, and contrast sets that are generated when a comparison is presented.