The present work is devoted to analysis of effective ionization process at early discharge stages in air and oxygen as the first necessary step before analysis of ionization in heated and moving air mixtures. It shows that there is no, so called, "threshold" of ionization in these gases at E/N ∼100·10^-17 V·cm² (30 kV/cm at atmospheric pressure), E-electric field strength, N- density of neutrals. Effective ionization is connected with the detachment processes of electrons from negative ions; it takes place at low E/N values, 40·10^-17 < E/N <200·10^-17 V·cm². Obtained values of effective frequency for MW discharges in air are in reasonable agreement with some data of experiments. Effective ionization rate constants in air MW discharge are higher than those in the glow discharge. Introduction.