A new method to measure the diffusion of adsorbed molecules over macroscopic distance on single crystal surfaces is presented and demonstrated with the system CO on Pt(111). A controlled coverage of molecules, typically of the order of 0.01 monolayer, is deposited on a well-defined small spot of the crystal surface. The specular intensity of a helium beam reflected on the same surface spot is recorded as a function of time in order to follow the decrease of the average coverage due to the diffusion of the molecules. The data extracted from measurements made between 273 and 373 K are in agreement with an Arrhenius law corresponding to an activation energy E/sub D/=6+or-0.3 kcal/mol and a pre-exponential factor D/sub 0/=(5.0+or-1)*10/sup -7/ m/sup 2//s.