This study investigated changes in the martensite start temperature (M s ) of NiTi-based shape memory alloys as a result of second phase precipitation. Precipitation of the second phase leads to a change in the matrix chemical composition and may alter the electron concentration (c v ) of the matrix. When the electron concentration of the matrix increases, the transformation temperature decreases, whereas the M s temperature rises when the c v of the matrix decreases. Additionally, if the precipitation does not result in a significant variation of the c v of the matrix, the variation in M s is small (<15°C). In the present work it is shown that alteration of the M s temperature as a result of precipitation is mainly due to the change in electron concentration of the matrix. The extrinsic effect of the precipitates on M s is also discussed on the basis of their quantities in the as-quenched and aged microstructures and the strain energy induced in the matrix.