In a recent paper, Seung-Bok Choi, Dong-Won Park and Suhada Jayasuriya (Automatica, 30, 899-904, 1994) have presented a stepwise time-varying sliding surface for variable structure control of a class of second-order uncertain systems. As the surface does not truly guarantee insensitivity of the systems to parameter variations and external disturbances, in this comment we propose a continuously time-varying surface that allows faster tracking and really guarantees robust behaviour of the systems.