Gas-liquid countercurrent flow tests were performed as part of the Semiscale Blowdown and Emergency Core Cooling (ECC) Project. These tests included: (a) preliminary tests to establish the lower plenurn liquid levcl for subsequent tests and to investigate bypass flow phenomena; (b) steady state tests to investigate the relation between steam flow and ECC entrainment; and (c) transient tests to investigate the effect of steam generation and high wall heat flux on countercurrent flow behavior. The tests were conducted using the semiscale vessel, a phase separator, and a storage tank. Resl~lts are presented as dimensionless volumetric flux graphs, computer printout of digital data at discrete time intervals, and computer-produced graphs ot anal6g data as a function of time. Descriptions of the test facility, instrumentation, operating procedures, and test conditions are presented. Rlaximurr~ errors are identified .and an error analysis is presented for the volumetric flux presentations. The purpose of this document is to present the data with minimum interpretation for use by the nuclear community and for support of subscquent interpretative analyses.