In this short note, the order reduction results of Petzold [SIAM J. Numer. Anal. 23(1986), pp. 837-852] for implicit Runge-Kutta methods, applied to index 1 differential/algebraic systems are extended to include a larger class of methods. The relationship between the order reduction results for differential/algebraic systems and recent results for stiff systems of ordinary differential equations is explained.