This work presents an experimental investigation of slug velocity in horizontal air-water flow. The velocity of gas-liquid interfaces was measured by cross-correlating the phase density function time series sampled by a pair of single-fiber optical probes. The superficial velocity of the liquid ranged from 0.6 to 2 m/s, while that of the gas ranged from 0.3 to 8 m/s. The probes were moved by micrometric screws, so that the vertical velocity profile was measured. The results indicate that the velocity profile changes according to the flow regime: in particular, a subregime was shown to exist between the plug flow and the slug flow.