The Hebb repetition effect refers to the finding that immediate serial recall is improved over trials for memory lists that are surreptitiously repeated across trials, relative to new lists. We show in four experiments that the Hebb repetition effect is also observed with a complex-span task, in which encoding or retrieval of list items alternates with an unrelated processing task. The interruption of encoding or retrieval by the processing task did not reduce the size of the Hebb effect, demonstrating that incidental long-term learning forms integrated representations of lists, excluding the interleaved processing events. Contrary to the assumption that complex-span performance relies more on long-term memory than standard immediate serial recall (simple span), the Hebb effect was not larger in complex-span than in simple-span performance. The Hebb effect in complex span was also not modulated by the opportunity for refreshing list items, questioning a role of refreshing for the acquisition of the long-term memory representations underlying the effect.Keywords Immediate serial recall . Complex span task . Working memory . Long-term memory Half a century ago, Donald Hebb (1961) asked participants in Tests of immediate serial recall are routinely used to investigate short-term or working memory, also known as primary memory (from here on we will use the term working memory). Most tests of serial recall involve the simple-span procedure, in which people must recall a list of items immediately upon presentation in forward order. Because of its limited capacity, working memory is commonly assumed to hold only the current list, perhaps with a few traces of the immediately preceding one, but it is not thought to be suited to acquire a representation of the commonalities of lists spanning four trials. Therefore, the pervasive Hebb effect documents the contribution of some longer-lasting form of memory, referred to as long-term memory or secondary memory, to tests of immediate serial recall. The Hebb effect implies that lists maintained for immediate recall leave long-term memory traces, and that these traces are used in immediate recall (Burgess & Hitch, 2005Page & Norris, 2009).Here we investigate whether the Hebb repetition effect is also observed with two variants of the complex-span paradigm. The typical complex-span task differs from the simple-span task by the addition of a distractor task that is to be carried out in between pairs of list items during encoding; here we also investigate a less common variant in which distractors are interspersed between items at retrieval. The distractor task usually requires processing without any explicit memory demand, for instance reading sentences (reading span, Daneman & Carpenter, 1980), solving arithmetic an experiment to remember lists of random digits for immediate recall in the order of presentation. Unbeknown to the participants, Hebb presented the same list on every third trial, interspersed with new random lists in the intervening two trials. Across 24 tria...