[en] Experiments have been conducted on probes for determining the secondary side quality within tube bundles which simulate the two-phase flow heat exchangers of pressurized water reactor plants. The testing was performed with a steam-water mixture of known quality flowing around unheated horizontal and vertical tubes. The probes operated by extracting low quality samples and indicating the bulk-stream's quality via relative concentrations of a chemical tracer. The tests showed satisfactory probe performance for indicated qualities up to 0.25 for the vertical tube simulator and up to 0.40 for the horizontal tube simulator