Several Pb** plus defects possessing the so-called laser-active Pb** plus (1)-type structure are examined by ESR in X-irradiated KCl:PbCl//2 and RbCl:PbCl//2 crystals. The essential core of these centers consists of a substitutional Pb** plus ion associated with a nearest neighbor anion vacancy along a LT AN BR 100 RT AN BR direction. Small perturbations in the vicinity of this core in the form of a charge compensating cation vacancy or a neighboring Pb**2** plus -cation vacancy complex generate a large number of Pb** plus (1)-type defects, which are similar in KCl and RbCl. Different for both hosts, however, is the influence of oxygen on the production of some centers. In the RbCl crystals the presence of the oxygen ions modifies the structure of one of the defects and induces the production of two supplementary centers, while no effect of the O**2** minus ions is observed on the Pb** plus (1)-type defects in KCl crystals.