We investigated by diffuse reflectivity the effect of hydrostatic pressure (1-1800 bar) on the thermal spin transition of Fe (PM-BiA) 2 (NCS) 2 polymorph I, where PM=N-2 -pyridylmethylene and BiA=4 -aminobiphenyl. We evidenced the onset of a progressive transformation into a phase of higher cooperativity which cannot be assigned to polymorph II. This result is discussed with respect to previous pressure investigations. © 2009 American Institute of Physics.