Films of EuS have been doped with up to 34-at. % Tb by coevaporation. From lattice constant measurements we conclude that the films are defect rocksalt structure solid solutions with the general formula Eu1-xTb xS1-yD'Alembertian signy, where D'Alembertian sign represents a vacancy. The magnetic properties change drastically between undoped films and the Tb substituted samples. The Curie point increases from 20 to about 135 K and the coercivity increases from 60 Oe to about 4 kOe in the x=0.34 sample. Our measurements indicate an increase in RKKY exchange because of the higher carrier concentration caused by Tb doping. The increased carrier density causes a decrease in resistivity of at least two orders of magnitude to about 10-3 Ω cm, the same order of magnitude as reported for bulk solid solutions of EuS-GdS. The peaks in the magneto-optic Kerr spectrum of EuS at about 2 eV are decreased in amplitude, broadened and shifted in energy with Tb doping.