We point out the importance of intensity-dependent refractive-index effects when measuring transmission or absorption characteristics of gases with high-power TEA CO(2) laser pulses. An example is given showing how self-focusing in low-pressure SF(6) gas severely affected the transmission of a Gaussian beam. This effect increased rapidly with pressure but remained unchanged when the TEA CO(2) fluence varied from 0.3 to 1.0 J/cm(2).