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Planning/implementation intentions has received support as a post-motivational construct across many behavioural domains but limited research has integrated the construct into a theoretical structure. The theory of planned behaviour (TPB) seems most suited to integrate implementation intentions into its structure because it lacks a planning construct. Still, semantic ambiguities between intention and planning may make the measurement domains of these proposed constructs similar even if the two constructs are conceptually different. The purpose of this study was (1) to evaluate the discriminant validity of motivation items (e.g. exert effort, try hard), intention items (e.g. intend, plan), and implementation intention/planning items (e.g. specific plans), and (2) to then integrate distinct motivation and planning constructs into the TPB model.The study followed a prospective survey design where TPB and related variables were measured at baseline and physical activity was measured two weeks later.Participants were 230 undergraduate students (70% female) who completed measures of the TPB and self-reported physical activity.Structural equation modelling showed that motivation and planning items possessed discriminant validity (p<0.01), but intention items could not be discriminated from motivation or planning (p>0.05). Still, planning did not augment motivation-physical activity relations (p>0.05).Intention items appear to straddle the measurement domains of planning and motivation. Careful item choices should be made when researchers wish to incorporate motivation and planning constructs in the same model. Planning did not add to the TPB model when predicting physical activity. The discrepancy between these results and prior research is discussed in terms of the differences between experimental and survey designs, and behaviour adoption versus maintenance.