The effects of the cis and traits form of the 4-OMe-phenylazobenzene at 1 % and 7% mole fraction on order, mesophase stability and dynamics of the 5CB liquid crystal were studied using the ESR spin probe technique. The presence of the trans azoderivative induced a downshift of the nematic- isotropic transition temperature T-NI which increased with the concentration but did not change the dependence of the order parameter on the reduced temperature. The cis isomer caused even larger TNI shifts, determining a marked decrease of the order parameter with a peculiar dependence on the reduced temperature, quite different from the standard Haller-type behavior. The spin probe dynamics appeared instead to be essentially unchanged by the presence of the azo-derivative.