A method (and corresponding apparatus) for downhole fluid analysis of petroleum formation fluids. The method includes capturing in a chamber of a downhole tool at least two immiscible formation fluids in a generally segregated state (the fluids including petroleum), activating a fluid mixing means to mix the fluids in the chamber to create an emulsion therefrom, and allowing the emulsified fluids to segregate while measuring light transmittance through the segregating fluids in order to calculate a transition time period based on the light transmittance through the fluids in the chamber. The transition time period is preferably bounded by the time required for the light transmittance values measured by the light detector to reach a baseline light transmittance. The transition time period characterizes the stability of an emulsion formed by the captured fluids. The methods and apparatus can also be used for other fluid testing applications beyond downhole formation fluid testing.