Potassium titanyl phosphate (KTP) has been shown to be a premier material for optical frequency second harmonic generation (SHG). Monovalent cations covering a large size range, e.g. Na+, Ag+, T1+, Rio+, and Cs+, have been used to partially or completely replace K+ in KTiOPO4 or KTiOAsO4, causing subtle structural variations which are frequently accompanied by marked changes in the nonlinear optical properties of these materials. The syntheses of (Na,K)TiOPO4 and (NH4,K)TiOAsO4, both by direct methods and by ion exchange with KTP and KTA are described here, and SHG intensities as a function of composition are presented. In the (Na,K)TiOPO4 phase space, powder SHG intensity initially remains constant with sodium content, then drops by a factor of ten when the KTP isostructure NaTiOPO4 is reached. In the (K,NH4)TiOAs04 system, SHG intensity is reduced by a factor of 5 between 40 and 50 mole percent NH4+. Single crystal X-ray data were collected on the compositions K.5(NH4).5TiOAs04 (KNTA) and NaTiOPO4 (NaTP). In KNTA, refinement reveals little change in the Ti coordination environment, however, unlike K.5(NH4).5Ti0PO4, no selective siting of the potassium and ammonium ions occurs. In NaTP, it is found that one of the Ti-O-Ti bond angles is reduced to 1300, and that one of the sodium atoms has the unusually low coordination number of four. KTixGa1-x0xPO4(F,OH)1-x is isostructural with KTP, with ▵(Ga-0)trans bond length differences of 0.056 and 0.027Å. Preliminary studies of the compositional dependence of SHG in the system KTixGa1-x0xPO4(F,OH)1-x show that SHG drops by 2 orders of magnitude between 50% and 60% Ga substitution.