Critical heat flux (CHF) of the fluoroketone fluid FKE-774 in vertical microgap channels is explored, with a focus on submillimeter spacings. Experiments were conducted using a 20 mm × 20 mm heated aluminum surface. Microgap channel spacings were decreased down to 0.3 mm, providing channel aspect ratios (height/spacing) as high as 67. In the limit where channel spacing is large, CHF was found to be 140 kW/m2 for saturated boiling at atmospheric pressure. A reduction in CHF of 55% was observed for the largest channel aspect ratio investigated. Results for degradation of the CHF limit with decreasing microgap spacing are compared to a correlation available in the literature and show a roughly hyperbolic dependence on channel aspect ratio (height/spacing) for aspect ratios larger than 10.