The crystallization rates of fused nifedipine in 6 mixtures with different polymeric pharmaceutical excipients were each measured as a function of the water content using an isothermal microcalorimeter. In all the excipients studied, the crystallization rate increased as the water content increased, but the extent of the increase in crystallization rate varied with the excipient. The spin-lattice relaxation rates (1/T₁) of deuterium oxide absorbed by the mixtures suggested that water mobility differed according to the excipient even if water content of the mixtures was the same. The crystallization rate of mixtures with crystalline cellulose, methyl cellulose, cornstarch and polyvinyl alcohol correlated with the 1/T₁ values. It is suggested that the difference in crystallization rate among the excipients can be explained by the water mobility which is governed by the interaction between water and excipient molecules. Increase in the mobility of water in the mixtures may decrease the nifedipine matrix viscoelasticity through the plasticizing effect of water, resulting in an increase in the crystallization rate. The plasticizing effect of water was confirmed by measuring nifedipine mobility in a sample without excipients. The nifedipine mobility, determined by measuring the spin-spin relaxation time of protons in fused nifedipine increased as the water content increased.