An image analysis technique is used to estimate the two-dimensional instantaneous densityfield of unsteady gravity currents produced by full-depth lock-release of saline water. Anexperiment reproducing a gravity current was performed in a 3.0 m long, 0.20 m wide and0.30 m deep Perspex flume with horizontal smooth bed and recorded with a 25 Hz CCD videocamera under controlled light conditions. Using dye concentration as a tracer, a calibrationprocedure was established for each pixel in the image relating the amount of dye uniformlydistributed in the tank and the greyscale values in the corresponding images. The results areevaluated and corrected by applying the mass conservation principle within the experimentaltank. The procedure is a simple way to assess the time-varying density distribution within thegravity current, allowing the investigation of gravity current dynamics and mixing processes.