Abstract Samples of tungsten and tungsten–tantalum alloy (with 5 mass per cent of Ta) were exposed to high-flux deuterium plasma at different fluences. The surface modification was studied with scanning electron microscopy, and deuterium retention was measured by thermal desorption spectroscopy (TDS). In the high fluence range of ∼3.5 × 10²⁶–10²⁷ m^-2, multiple large-size blisters are formed on the W surface, while blisters on the W–Ta surface are considerably smaller in size and number. Deuterium retention in this fluence range was found to be systematically higher in W than in W–Ta. Correlation between the evolution of the blistering patterns and the TDS spectra as a function of fluence suggests that trapping in the sub-surface cavities associated with blisters is the predominant trapping mechanism in tungsten in the case of high fluence exposures. We attribute the lower retention in W–Ta under the investigated conditions to the weaker blistering.