Ambient temperature plays an important role in the operational behavior of a vehicle. Temperature variances from 20 F to 72 F to 95 F produce different operation from different HEVs, as prescribed by their respective energy management strategies. The extra variable of Climate Control causes these behaviors to change again. There have been studies conducted on the differences in operational behavior of conventional vehicles as against HEVs, with and without climate control. Lohse-Bush et al conclude that operational behavior of conventional vehicles is much more robust as compared to HEVs and that the effect of ambient temperature is felt more prominently in HEVs (1). However, HEVs cover a broad range of powertrain architectures, climate control systems, vehicle weights etc.The objective of this paper is to examine three different HEVs under three different temperature conditions, both with or without climate control, and come up with observations and trends on their energy usage and operational behavior. For the scope of this study, only the UDDS (Urban Dynamometer Driving Schedule) is used since this produces the most observable trends vis-à-vis fuel usage and battery energy usage. The three vehicles in this study are the 2010 Honda Insight, the 2010 3rd Gen Toyota Prius, and the new 2013 VW Jetta HEV. An additional parameter of whether cold starts show any differences compared against hot starts is also examined. All tests are performed at Argonne National Laboratory's Advanced Powertrain Research Facility on the 4-WD Chassis Dynamometer. The tests are charge sustaining so that the total change in battery energy is less than 1% of the total fuel energy usage.The results have been examined from various angles. First the paper will go through the effect of climate control at 20 F and its relation to energy consumption at 72 F, followed by a similar observation of the effect of climate control for 95 F. The paper will then elaborate on hot start vs cold start for each temperature. This will be followed by a comparison of all 5 cases (20 F without or without climate control, 72 F and 95 F with or without climate control). This analysis is done for all three vehicles followed by a direct comparison of three vehicles across all temperature ranges. The crucial variables observed are Engine On time (EOT), Average Engine Fuel Power (AFP), Cumulative Fuel Consumption (FC), Engine Fuel Energy, and Instantaneous Fuel Power. A combined study of these variables will showcase the operational differences in the three vehicles at different temperatures to how their energy consumption varies from case to case.