The effect of water droplets on strained methane-air laminar flames is investigated using particle image velocimetry of both gas and liquid phases. We use the impinging flame configuration to measure the laminar flame speed (gas) as well as the motion of the liquid phase, simultaneously. Water droplets of mean diameter 36.6 µm are produced by an ultrasonic atomizer and dispersed in a methane/air flow, for a constant molar ratio (12–36%) to the fuel (methane). This corresponds to a water mass fraction of 0.8–2.2% at stoichiometry. The slip motion between gas phase and droplets is quantified by seeding 1.7 µm mean diameter oil droplets into the reactant flow, and using an image segmentation method to determine the velocity of the gaseous flow and water droplets separately. The result reveals a clear slip velocity between the two phases: the inertia of relatively large droplets results in a drift into the flame front at a higher speed than the gaseous flow, by a factor of 10–30% in most cases. Measurements of the gas-phase flow show that the addition of water droplets significantly reduces the reference flame speed, especially at high strain rates. However, numerical simulations on water vapor addition at the same conditions predict only a slight drop in the reference flame speed, suggesting water droplets are more effective in flame suppression than vapor. This is the first time that the two-phase PIV technique has been applied to solve a problem in combustion investigations. We validate this technique under a non-reacting impinging flow prior to the flame experiment, and report the strategies adopted for better image quality and subsequently easier segmentation between oil and water droplets. The technique can be next applied to measure the slip velocity for volatile fuel sprays, where the slip motion plays an important role in determining both their evaporation rate and residence time in a flame.