Apparent molar volumes, expansibilities, and adiabatic compressibilities have been determined by density and ultrasound velocity measurements, in the temperature range of 5-50 degrees C, of aqueous solutions of the diblock copolymer denoted P94E316, where E denotes oxyethylene, P denotes oxypropylene, and the subscripts denote block lengths. The critical micelle concentration (cmc) was determined at 20 degrees C from ultrasound velocity data by an analytical method based on the Phillips definition of the cmc; it was impossible to use the method at other temperatures due to the high or low values of the cmc. Critical micelle temperatures were derived for different concentrations from ultrasound velocity measurements applying the same procedure as that for the cmc.