Class A and H oil well cements are compared at 25 degrees and 50 degrees C with 0%, 1%, 2%, and 4% CaCl2. Up to 4% CaCl2 accelerated Class A thickening, but 4% led to slower thickening than 2% for Class H. C3S hydration in the two cements responded differently to CaCl2. CaCl2 always accelerated aluminate hydration. For Class A, CaCl2 accelerated early Ca(OH)(2) precipitation, but sometimes reduced the amount at longer times. This may be coupled to C-S-H gel composition changes. For Class H, Ca(OH)(2) precipitation changes nonlinearly with CaCl2 concentration. Ettringite to monosulfate conversion and Friedel's salt formation were sometimes seen.