Abstract ( P,T) phase diagram of isostructural NiMnSi n Ge 1- n and NiMn 1- t Ti t Ge systems were determined. For this purpose, in the range of hydrostatic pressure up to 1.5 GPa (1.8 GPa), the magnetic phase transition temperatures were measured using the ac susceptibility method and the investigation of the magnetization dependence on pulsed magnetic field were carried out. For samples with n=0.05 and t=0.05, similarly as in the case of NiMnGe, the positive correlated coupling of magnetic and structural phase transitions was observed. In the NiMnSi n Ge 1- n system, external hydrostatic pressure enhanced such changes of magnetic properties as the increase of silicon concentration. In particular, for the sample with n=0.2, a pressure enhanced discontinuous magnetic phase transition from helicoidal phase to the phase which exhibits macroscopic properties of noncollinear ferromagnetic order of F x AF z type (characteristic for the samples with higher silicon content) was observed. The experimentally obtained phase diagram was discussed, based on a simple thermodynamic model which presums a linear dependence of effective exchange integrals on relative changes of specific volume of crsytals.