A capacitance dilatometer is described which automatically records the height of a mercury column as a function of time. Changes as small as 10−4ml have been detected, with an instrumental stability allowing experiments of several days’ duration. The technique is based on the measurement of the capacitance of an aluminum foil‐covered, precision bore, glass column containing Hg. The dilatometer has been used to measure coefficients of expansion and rates of polymerization and hydrolysis.