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We have systematically investigated the magnetic properties of the rare-earth compounds PrCo8Si and PrCo3–xSix, the crystal structures of which are hexagonal CeNi3-type and rhombohedral Be3Nb-type, respectively. Sharp cusps were observed in the real and imaginary parts of the ac susceptibility at temperatures between 282 (x = .08) and 209 K (x = .23). Also observed were bifurcations of the zero-field-cooled and field-cooled magnetization curves at the same temperatures, showing that the magnetization was irreversible below these temperatures. The effect of Si substitution for Co in this system is to decrease the average ferromagnetic exchange interaction, thus decreasing the Curie temperature, without introducing significant random magnetism effects due to exchange fluctuations or random magnetic anisotropy.