A one-dimensional particle-in-cell/Monte Carlo code with three velocity components is developed to simulate a capacitively coupled radio-frequency Ar/H2 discharge at low pressure, and to investigate the effect of adding hydrogen to an argon discharge. This self-consistent kinetic simulation technique allows one to study fundamental processes in the discharge at the molecular level. It is shown that the addition of small amounts of H2 to an Ar discharge has profound effects on the discharge behavior, i.e., a change in the electron energy probability function, an increase in the electron density at low H2 content and a decrease at higher H2 content, as well as a dip in the Ar+ ion density in the center of the discharge at higher pressure. These effects can be explained by the collision processes taking place in the discharge. The simulations were carried out in the pressure range 50–250 mTorr, at voltages of 300 and 800 V, while the H2 content was varied between 0% and 10%, at a constant driving frequency of 13.56 MHz. © 2003 American Institute of Physics.