1. When compared to the master die, all five impression material specimens showed a shrinkage of from 0.07% to 0.23%. When used as substrates for electrodeposition, these dimensions partly predetermined the dimensions of the electrodeposits. (This will be discussed in Part II of this study). 2. When polymerized and maintained at room temperature, both President and Polyjel demonstrated a slight polymerization shrinkage then remained stable during a 3-week bench set. 3. When polymerized at higher temperatures, both President and Polyjel demonstrated considerable dimensional changes during the first hour as they cooled to room temperature then remained stable during the 3-week dry bench set. 4. President was little affected by a 5-hour immersion in an acid electroplating bath. 5. Polyjel, during 5 hours of immersion, tended to expand in both acid and alkaline electroplating baths.