Pigeonites undergo a P21/c-C2/c phase transition during compression and on increasing temperature. It is known (Arlt et al., 2000) that there is a direct relationship between the transition temperature and the cation population of the M2 site. The aim of this study was to constrain the combined effect of Ca, Fe and Mg on the transition pressure Pc, and imporve its prediction once the composition of the pigeonite is known. A sample of a P21/c pigeonite from the Pecora Escarpment Antarctic Ureilite PCA-82506-3 with composition Wo6En7Fs18 and free of exsolved augite was studied by mans of single crystal X-ray diffraction using a ETH-DAC at 14 different pressure values up to about 7.14 GPa and room-T. In the P21/c LP-phase the unit-cell parameters showed a non-linear decrease and almost isotropic linear axial compressibility with a ratio of beta_a:beta_b:beta_c = 1.00 : 1.35 : 1.01. A first order phase transition was found between 4.66 and 4.89 GPa associated with the disappearance of the b-type reflections (h + k = odd) and a strong discontinuity (about 2.5%) in the unit-cell volume. By fitting the pressure-volume data for the P21/c phase using a BM3 equation of state @S) the following EoS coefficients: Vo = 424.66(6) A3, KTo = 1 04(2) GPa and K' = 8(1) were obtained. Our data confirm what expected by the previous literature data (Arlt et al., 2000; Ross & Reynard, 1999; Nestola et al., 2004) and by the results recently obtained on the P21/c BTS-308 pigeonite. In particular i) the experimentally measured transition pressure Pc (4.66 GPa) is in perfect agreement with the value expected by the average ionic radius of the cation at the M2 site; ii) our value of KTo [4] compared with the available KTo values of PZll/c samples from literature (Ross & Reynard, 1999; Nestola ef al., 2004) c m h n s that the P21/c phase becomes softer with increasing the Fe content.