Barriers to bicycling may vary widely depending on individual, attitudinal, and built environment characteristics; barriers may be modest for some (e.g. requiring secure bike parking) or significant for others (e.g. improving regional bicycle-accessibility). This research suggests that for a substantial population of travelers who are interested in bicycling but unwilling to cycle regularly, barriers to increasing commute cycling may be different than for individuals who already commute by bicycle at least occasionally. Treating these two populations as one homogenous group may be inappropriate and reduce the effectiveness of bicycle promotion strategies. This research disaggregates these two prospective commute-cyclist populations and tests how attitudes, socio-demographics, and the built environment impact their commute mode choice. Socio-demographic and attitudinal data are drawn from a survey of “Bike to Work Day” participants in Denver, Colorado while built environment measures – including street network connectivity, street network density, and trip distance – were calculated with GIS. Bicycle commuting decisions within the two groups of prospective cyclists are estimated using binary and ordered logistic regression. Distinct socio-demographic and built environment factors are significant for different groups of prospective cyclists. Significant attitudinal variables are similar across groups; for both populations, convenience and utility of the bicycle relative to other modes is significant, suggesting that these factors outweigh issues regarding safety for the sample population. Findings from this research demonstrate that there are important distinctions between the decision to start commuting by bicycle and the decision to increase the frequency of bicycling to work.