The composition variation in Zn-Sn-O (ZTO) films grown by a pulsed-laser deposition technique was examined. The composition variation is attributed to Zn evaporation, which occurs at temperatures higher than 600˚C under low oxygen pressures. ZTO films grown at and below 500˚C are amorphous. Upon increasing the growth temperature of the ZTO films, the Zn evaporation was enhanced, and crystalline phases were observed. Textured SnO_2 films were finally obtained when the substrate temperature of the films was raised to 750˚C. Composition variation was also observed in post-annealed ZTO films grown by dc reactive magnetron sputtering. The post-annealing experiments indicate that a low oxygen pressure is the essential condition for Zn evaporation in ZTO films. The effect of the growth temperature on the structural, electrical and optical properties of the ZTO films was also investigated.