[en] Results are presented of work using transmission electron microscopy to examine the surface layer of molybdenum samples containing high levels of injected helium. At ion doses below the critical level for surface blister formation a very high density of small helium bubbles was observed over a wide temperature range. Of interest was the fact that these bubbles tended to align on a body-centred-cubic lattice thus enabling the use of electron diffraction to estimate the lattice parameter of the array and hence give bubble densities. At higher ion doses blisters were easily seen, though they were not found at irradiation temperatures above 1100K. These aspects are discussed within a framework of a mechanism for blister formation involving the breakaway coalescence of the high density of small bubbles at a critical volume swelling in the helium peak. In turn, this bubble coalescence leads to a rapid build-up of excess gas pressure thus creating the blisters. Theoretical values of critical helium doses based on this model appear to agree reasonably well with experimental values. (author)