Is attentional capture contingent on top-down control settings or involuntarily driven by salient stimuli? Supporting the stimulus-driven attentional capture view, Schreij, Owens, and Theeuwes (2008) found that an onset distractor caused a response delay, in spite of participants' having adopted an attentional set for a color feature. However, Folk, Remington, and Wu (2009) claimed that this delay reflects separate, nonspatial filtering costs instead, because the onset effects were additive with color-based capture effects, and capture should have caused underadditivity. The present Experiment 1 shows that contingent capture caused by additional color cues is also additive, just like the onset effect. This makes additivity a dubious diagnostic with regard to spatial capture. Experiment 2 demonstrates that it is possible to obtain underadditivity when attention-demanding distractors have sufficient capturing power. Experiment 3 shows that the abrupt onset interference turns into a benefit when the locations of the onset and the target coincide. Together, these results argue in favor of stimulus-driven attentional capture by abrupt onsets.