Non-linear behaviour of NbN and YBa₂Cu₃O_7-x films at microwave frequencies is considered. Basic non-linearity mechanisms in wide superconducting films and the corresponding cut-off frequencies are discussed. Non-linear microwave properties of the films are analysed on the basis of dynamic current-voltage characteristics derived from the film response to short current pulses. The mixing effect is observed on superconducting NbN and YBa₂Cu₃O_7-x thin-film microbridges and coplanar lines. The intermediate-frequency conversion loss is analysed as a function of the local oscillator power. The experimental data are compared with simulation results.