There is a growing trend among consumers to serially consume small, incomplete “chunks” of multiple media types – television, radio, Internet and print – within a short time period. We refer to this behavior as “media multiplexing” and note that key challenges for integrated marketing communications (IMC) media planners are (1) predicting which media or combination their target audience is likely to consume at any given time and (2) understanding potential substitutions and complementarities in their joint consumption. We propose a forecasting model that incorporates media-multiplexing behavior of both traditional and new media, their interdependencies, and consumer heterogeneity and calibrate the model using a rich database of individual-specific media activity diaries. The results suggest that accounting for media synergies within a single utility specification significantly improves model forecasts. We also introduce a utility function that directly models cross-channel media complementarities via interactive effects of the satiation parameters of own and joint consumption of various media types. Finally, our individual-level analyses generate unique insights on consumer-level media switching, multiplexing and individual heterogeneity often ignored in aggregate data.