The Spanish intransitive motion verbs subir 'go up' and bajar 'go down' variably appear with the middle marker se. In this study, we examine aspectual (telicity, punctuality) and pragmatic (counter-expectation, speaker involvement) analyses of se-marking. Adopting the variationist method, we operationalize and test hypotheses via quantitative analysis of variation between the middle se-marked and non-se-marked forms in corpora of spoken and written Mexican and Peninsular Spanish. Variability in se-marking is structured, as shown by recurrent patterns reflected in frequencies of co-occurrence. We adduce evidence that factors in speakers' choice of the middle se-marked form include a focus on the moment of change (operationalized by co-occurring locative preposition), the foregrounding of the situation (clause type), the topicality of the subject (subject expression), and speaker involvement (grammatical person, tense-mood-aspect); that is, a confluence of aspectual and pragmatic factors. Multivariate analysis further reveals that se-marking is most likely to occur in a particular 'enter-exit a vehicle' subir-bajar construction. A number of other constructions with clear se-marking preferences also emerge from the data. Rather than generalizations about abstract structures, the empirical results reveal usage-based constructions of differing degrees of lexical specificity and fixedness. A usage-based constructions approach allows us to dispense with a grammar-pragmatics dichotomy. (c) 2008 Elsevier B.V. All rights reserved.