Abstract The Thiokol and silicone impression materials were investigated in a laboratory study to determine their best means of application to removable partial denture impressions. Several methods were evaluated and a technique which produced reliable, consistent results was evolved. A clinical situation was selected and casts made with Thiokol and silicone material were compared with irreversible hydrocolloid, reversible hydrocolloid, and silver-plated Thiokol casts. The reproducibility of casts from each impression material was statistically compared using data obtained on the microcomparator. By means of graphs of this data the models were compared with each other. All of the casts were compared with a casting which fit the mouth.