PurposeIn the American National Standards Institute/American Dental Association specification no. 19, compatibility of impression materials with dental stones is assessed by the presence of a 20μm‐wide line reproduced on an unmodified calcium sulfate dihydrate cast. In actual dental practice, modified type IV dental stones are used, although little is known of their compatibility with polysulfide impression materials. This study evaluated the compatibility of 6 polysulfide impression materials and 11 modified type IV dental stones.Materials and MethodsA line 20 μm wide was etched on four glass dies. Four samples of each combination of impression material and dental stone were prepared according to the manufacturer's directions with an additional 3 minutes for the final setting time. Compatibility was determined by the presence of the reproduced line on the dental stones, as observed under low angle 10 × magnification by four rater groups.ResultsThe line was reproduced on all of the impression specimens, and the examiners recorded 66 positive identifications of the line on the stone casts out of a possible 1,056 ratings for a total of 6.25% of the specimens. Out of a possible 66 impression‐stone combinations, only 18 reproduced the 20‐μm line. The combinations reproducing the lines most frequently (75%) were Neoplex with Blue Die Stone (Columbus Dental, St Louis, MO) and Coeflex with Indic Die Stone (Coe Lab Inc, Chicago, IL).ConclusionsThe study showed that many combinations of polysulfide impression materials and modified type IV dental stones did not reproduce the 20‐μm line; therefore, not every polysulfide is compatible with every type IV