Abstract: A process for the manufacture of single crystals of MTiOXO.sub.4, wherein M is K, Rb or Tl and X is P or As, of optical quality and of sufficient size for use in nonlinear optical devices. In the process, starting ingredients chosen to be within the region of the ternary phase diagram in which the desired crystal MTiOXO.sub.4 product is the only stable solid phase, are heated to produce MTiOXO.sub.4 and then controllably cooled to crystallize the desired product. In a preferred method the ingredients are placed in a temperature gradient in which the hot zone ranges from about 800.degree.-1000.degree. C. and the cold zone about 10.degree.-135.degree. C. cooler, maintaining the temperature gradient for about 3 days to about 3 weeks, cooling, and recovering MTiOXO.sub.4 crystals.