The magnetic properties of the ThFe11Cx (x=1.5 and 1.8) compounds have been investigated at ambient and high pressures. The thermal evolution of the saturation magnetization has been determined up to 550 K. These compounds are found to have a reorientation of their easy magnetization axis, which is aligned along the tetragonal c axis at room temperature but lies within the basal plane at 4 K. The reorientation of magnetization results from competing magnetic anisotropy of inequivalent iron sites. The spin reorientation temperatures are 257 and 202 K for ThFe11C1.5 and ThFe11C2, respectively, showing a significant effect of the carbon concentration. The anisotropy parameters have been obtained on both sides of the spin reorientation transition in the 4–300 K temperature range. The effect of pressure on the spin reorientation temperature has been investigated up to 10 kbars. This spin reorientation transition is found to decrease significantly upon increasing pressure with rates of -9.4 and -8 K/kbar for ThFe11C1.5 and ThFe11C1.8, respectively. A magnetic phase diagram is proposed for both ThFe11Cx compounds.