An experimental study into the sheet breakup mechanisms of two direct-injection gasoline pressure-swirl atomizers has been performed under conditions of varying fuel pressure and injector back pressure. A novel method of illumination, which allows a single thickness of the hollow-cone spray to be examined, has been developed. The results of the study indicate that there are three major mechanisms contributing to the ultimate disintegration of the liquid sheet, namely, aerodynamic instability, perforated sheet breakup, and air-core induced disturbances. The relative importance of the breakup mechanisms is shown to depend on atomizer operating condition and nozzle geometry. Sheet breakup lengths determined experimentally by a void-fraction analysis were compared with the predictions of three breakup models from the literature. The comparison suggests that the models underpredict the breakup length under all but substantially subatmospheric back-pressure conditions. The results also indicate that special care is required when using existing breakup models where atomizers with offset-nozzle geometries are used.