A phenomenological modelling approach has been developed for the description of die massive and lamellar microstructures which form in gamma-TiAl alloys at high and moderate cooling rates, respectively. The nucleation of both massive and lamellar gamma is described by classical heterogeneous nucleation theory which takes into account that nuclei are formed predominantly at a grain boundaries. The thickening rate of the lamellae is described with a modified multicomponent Zener model of precipitation while the growth rate of massive gamma is calculated with an expression for interface-controlled reactions. The model, which is coupled with a thermodynamic database, permits to investigate the influence of the alloy composition and cooling rate on the proportion of massive and lamellar gamma. Calculted CCT diagrams showed good agreement with experimental data for binary alloys. The approach was also used to discuss the effect of Nb additions on the transformations kinetics and on the competition between lamellar and massive microstructures. The calculations illustrate how Nb additions tend to favour the massive microstructure by slowing down the growth of lamellar precipitates.