It is shown that control of the phase of calcium carbonate produced during hydrothermal crystallisation can be achieved by the use of titania as a catalyst. In this work it was demonstrated that if either amorphous or the rutile phase of titania is added the aragonite polymorph of calcium carbonate is produced to the exclusion of all other phases. When anatase is the major titania form present then the calcite phase of calcium carbonate is favoured though some aragonite is still produced. The selectivity towards calcite is enhanced by increasing the hydrothermal processing temperature from 120 degreesC to 135 degreesC. It is suggested that the titania phase present catalyses the crystallisation by providing nucleation sites for the calcium carbonate to grow from. (C) 2003 Kluwer Academic Publishers.