The process of the high spin (HS)<-->low spin (LS) transition is studied in the polycrystalls of the Fe(III) thiosemicarbazonates, M[Fe(Th-R-Sa)2], with M, R = Na, H (A); NH4, 5-Br (B) and K, 5-Br (C) by the EPR method at atmospheric and hydrostatic pressure up to 600 MPa in the 80–400 K temperature range. The χ and  µeff were also measured for T =1.8–400 K. Spin transition (ST) in A is a continued one:  µeff changes from 5.9 µB at 330 K up to 5.7 µB at 50  K whereupon changes sharply to 3.4 µB at 1.8 K . The analysis of the EPR line width ∆B has shown its exchange origin. LS complexes (LSC) are not statistically distributed among the HS ones (HSC), but are gathered in a limited regions of structure – domains. The density of the LSC in domains increases with the T lowering. (∆B changes from ~ 70 – 80 mT at 200 K down to ~  20 mT at 80  K ). The line width ∆B sharply changes in the ST process at the two T intervals (240 – 236  K and 195 – 191  K ), pointing to the redistribution of the LSC in domains. Within 195 – 191 K this phenomenon is accompanied by the sharp increase of the LSC quantity.

The application of the external pressure P stimulates ST, however the character of the (HS)<-->(LS) process depends on T . At T>240 K LSC are not distributed accidentally, nevertheless their density at the regions of gathering is not high (∆>50 mT). The application of P below 236 K destroys domains partly, a number of the LSC returns to the HS state. The domains reorganization occurs at increasingly high P values at the T lowering.

ST takes place in B at the temperature interval 100 – 250 K (in interval 150–200 K it occurs more rapidly). It occurs through the domain formation as well but, contrary to the A case, the density of the LSC in domains grows very quickly. Already at the beginning of the EPR registration at Patm  ∆(240  K )  ~  21 mT; ∆(170 K ) =~ 13.7 mT. In the T interval corresponding to the equal LSC and HSC contents, at T ~ 140 K, the abrupt transition from the separate existence of the LSC and HSC phases to their distributed state takes place. The pressure leads to the ST as well as the temperature does. About half of the complexes passes to the new spin state at P max  = 600 MPa. The state of the crystal is a transitional one at this P region. It shifts to the lower pressures at the temperature lowering. ST in C occurs in the 360 – 250 K T range. Its study allows to understand better the role of T and P at the ST process.

The processes of the domain formation and destruction show a definite hysteresis. The increase of the LSC density at the pre-domain state is defined by the pair correlations of the LS complexes. The cooperative effects lead to the sharp transformation of domains. The features of the self-regulation are inhered to the process of the transition of the crystal with the LS complex domains to the state equilibrium at given T and P .