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Attentional capture occurs when salient stimuli divert attention independently of an observer’s goals. Past research has established that stimuli which appear suddenly – sudden-onsets – reliably capture attention, while other features such as a unique colour among homogeneous distractors do so much less convincingly if at all. These conclusions have been reached on the basis of research in which observers focus their attention on a single task, so questions have been raised about their applicability in more ecologically valid situations in which the simultaneous performance of more than one task may compromise the availability of attention. The present research investigates the specific suggestion that in dual-task situations sudden-onsets capture attention less effectively and unique colours capture attention more effectively than they do under single-task conditions. Across twelve experiments, the research focused on the ability of colour singletons, colour-changes, and sudden-onsets to capture attention in a range of different contexts in which concurrent task load and stimulus salience were manipulated. In contrast to most past research in the area, the effects of manipulations of temporal as well as spatial uniqueness were considered in the context of the same search task throughout. This allowed for a more thorough assessment of attentional control in displays containing the singletons of interest and a more rigorous direct comparison of their effects. As has been seen in a single-task context, there was a distinction between effects of capture due to sudden-onsets and uniquely-coloured stimuli in a dual-task context. Capture by a uniquely-coloured item seems to rely on top-down processes and the enduring salience of its unique feature, while capture by a sudden-onset seems more related to the transience of its unique feature and the bottom-up processes which enable the sensory registration of visual information. Of the hypotheses that are considered, these findings are most consistent with the transient hypothesis, which emphasises the importance of the sustained uniqueness of a stimulus in dual-task conditions.