Single crystals and polycrystalline samples of transparent calcite (Iceland spar) were used to investigate the effects of pressure (p), temperature (T), pressure beyond an accepted equilibrium value (Δp), and grain size (D) on the kinetics of the calcite–aragonite transformation. Transformed mass fractions x(t), produced after different times of exposure to constant pressure ranging from 14 to 25 kbar (1.4 to 2.5 GPa) and constant temperature from 300 to 600 °C, were determined from integrated X-ray peak intensities of the (012), (021), and (111) reflections of aragonite. Values for the rale constant K and the exponent n characterizing the transformation were computed from the x(t) data using Cahn's nucleation and growth model for solid–solid transformations. At 17 kbar(1.7 GPa), K(s−1) increases from 2.00 × 10−5 at 300 °C to 1.39 × 10−3 at 600 °C. The exponent n, of the order of 1, mostly <1, indicates that the nucleation stage is terminated rapidly by site saturation and that most of the transformation takes place thereafter by growth as expected from theory. For single crystals the relationship between x(t) and Δp, for a period of 1 hour is almost linear. At 600 °C the relative increase of x(t) amounts to 0.1 kbar−1 (1 × 107 Pa−1). For a given time, x increases nearly exponentially with T. The apparent activation energy for the transformation, at 17 kbar (1.7 GPa), is 16 kcal mol−1. For polycrystals x(t) decreases as . This somewhat surprising result may be related to deviatoric stresses and stress concentration by the already transformed volume fractions, which act as misfitting, ellipsoidal, inclusions.