The Texaco Gasification Process employs a high temperature, high pressure slagging gasifier, in which the viscosity of the slag plays a key role in determining operating conditions. Empirical slag viscosity models available in the literature, as well as prior laboratory testing have generally concentrated on low titanium slags/ashes. During the gasification of waste materials, titanium dioxide is an important compound with respect to ash and slag behavior. In the present study, slag viscosity was measured under reducing conditions between 1150 and 1500 degrees C on FeO-CaOAl2O3-SiO2 based slags containing up to 30 wt.% added TiO2. Most of the titanium-rich slags exhibited viscosity behavior characteristic of crystalline slags, with critical viscosities ranging from similar to 1230 to similar to 1380 degrees C. Crystalline phase analysis of the slag samples reveals that titanium dioxide (as well as other phases) readily nucleate, and that the residual glass phase generally consists of CaO +/- TiO2 +/- Al2O3 +/- FeO silicate glasses having low melting points. Thermochemical modeling of the titania bearing slags using fractional crystallization theory shows some promise for predicting the observed crystalline phases and residual melt compositional evolution. Accurate activity coefficients of high temperature slag components, nucleation and growth kinetics information of crystalline phases, and thermodynamic data for several unusual species discovered by electron microprobe analysis are needed, however, before computer modeling can be expected to accurately predict slag behavior.