This paper reports an experimental and theoretical study of composition and temperature dependence of alpha(2) phase decomposition in lamellar gamma-Ti -(43 similar to 47)Al-(4 similar to 10)Nb alloys. The alpha(2) phase decomposes to nano-sized orthorhombic (O) phase in the alloys (the Nb content >= 5.5 at.%) at temperatures of 550 similar to 750 degrees C. The transformation temperature decreases with increasing the Al content, but increases with increasing the Nb content. The Nb partitioning coefficient between O and alpha(2) typically equals to 2, and decreases with increasing the Al content and temperature, confirming that the O phase transformation is controlled by Nb diffusion. The alpha(2) to omega(0) phase transformation takes place in the alloys (the Nb content > 7 at.%) at 800 degrees C. The blocky omega(0) phase is enriched in Nb and the Nb partitioning coefficient between omega(0) and alpha(2) is about 1.3, indicating that the omega(0) phase transformation is also related to Nb diffusion. The pseudo-binary phase diagram calculated by first-principles correctly predicts the alpha(2) to O phase transformation at temperature below 750 degrees C and alpha(2) to omega(0) phase transformation at temperature above 750 degrees C in the alloys. Since the alpha(2) phase is unstable thermodynamically at intermediate temperature, such kinds of alpha(2) to O and omega(0) phase transformations are considered necessarily for design of high Nb-containing gamma-TiAl alloys.