Steam condensing in vertical annular passages experiences regular flow and pressure oscillations. An experimental program with a counterflow condensing heat exchanger with flow visualization was designed to obtain the flow and pressure data along with photographic and video records of condensing steam in vertical upflow in annular passages. The experimental results were compared to the literature on condensation oscillations. From the observations and data collected in the experiment, a 3-region physical model of the condensing process was developed. This model contained a region of condensing annular flow, an interfacial region of slug and bubble flow, and a region containing subcooled condensate. It had been expected to find test conditions that would yield a stable condensing flow. However, flow and pressure oscillations were observed at all test conditions. While these oscillations were not particularly regular and sinusoidal, spectral analysis of the data revealed a 2-2.5 Hz oscillatory frequency. The observed flow and pressure oscillations were indicative of oscillations unique to condensing passages previously observed in condensation experiments that used a refrigerant as the working fluid.