The catalytic effects of small amounts of water (similar to 10(-3) mol/l) added to the ethylbenzene oxidation reaction with molecular oxygen catalyzed by the system {Fe(III)(acac)(3) + 18-crown-6 (18C6)} were revealed. An increase in the reaction rate was observed, and the direction of the oxidation reaction changed. In the presence of the catalytic system {Fe(III)(acac)(3) (5.0 x 10(-3) mol/l) + 18C6 (5.0 x 10(-3) mol/l) + H2O (3.7 x 10(-3) mol/l}, the main product at early stages of ethylbenzene oxidation is acetophenone, rather than alpha-phenylethyl hydroperoxide produced in the case of catalysis by the system {Fe(III)(acac)(3) (5.0 x 10(-3) mol/l) + 18C6 (5.0 x 10(-3) mol/l)} (in the absence of water admixture). A method is proposed to evaluate the activity of forming in situ (Fe(II)(acac)(2)) (x) (18C6) (y) (H(2)0) (n) complexes at microstages of chain initiation (O-2 activation) and propagation (Ct+RO2 center dot) involving the catalyst, which determine the rate and selectivity of oxidation. A possible "dioxygenase-like" mechanism of conversion of iron complexes with 18C6 in the presence of activating H2O admixtures is discussed.