In general, the beam quality of industrial high-power CO2 lasers does not reach the maximum theoretical value, but it is dependent on the operation parameters of the particular system. This is due to the distortion of the intracavity wavefront which results from the non- homogeneity of discharge and gas flow and from the power absorption of mirrors and windows. For this reason, investigations have to be done in order to identify and then to minimize or to compensate the distorting effects caused by basic `elements' of a fast axial flow rf-excited laser. Conventional linear electrodes, improved helical electrodes, with or without a discharge-stabilizing air gap between electrodes and quartz tube, have been compared with respect to the minimum achievable phase distortions. Bending mirrors, resonator mirrors and outcoupling windows absorb a part of the beam power and of the uv-luminescence of the discharge and, therefore, become deformed. The temperature of the quartz tubes changes with laser operation time leading to a transient thermally induced lens in the laser active medium. This effect in combination with the time constants of mirrors and windows determines the warm-up period of a laser system. The detrimental transient effects can be reduced either by optimization of the basic elements of the system or by inserting optical elements which can be used for active compensation. One example is the use of an intracavity adaptive mirror offering the possibility to alter the mirror surface curvature. An experimental high-power CO2 laser has been used to demonstrate and to verify approaches to minimize phase distortions caused by the effects mentioned above.