The time of equilibration, ($) over bar t, in reversible aggregation of colloidal particles was investigated. Estimates were calculated from Smoluchowski-type kinetic equations, extended to include dissociation reactions. An extension of the analytic solution for the monomer-dimer system yields a closed form expression for ($) over bar t in the general case. Equilibration time can also be estimated from a single equation which expresses the rate of change of the number of composite particles, using averaged values for the rate constants. Sodium-induced aggregation of phosphatidylserine vesicles was studied by means of turbidity and energy transfer measurements. The reversibility of the aggregation process has been verified with both procedures, confirming also no exchange of the fluorescent probes used between vesicles in an aggregate. The calculations can predict the decrease in ($) over bar t values with an increase in vesicle concentration and give rough estimates for ($) over bar t values. The kinetics of turbidity increase is similar to that of fluorescence increase of acceptor molecules. With 200 mu M lipid + 650 mM NaCl at 23 degrees C, the turbidity and fluorescence intensity reach equilibrium values in approximately 2 min. At lower temperatures both the extent of aggregation and ($) over bar t increase in accord with estimates. At 35 degrees C the extent of aggregation decreases as expected, but ($) over bar t increases. We interpret this to be the result of an increase in the potential barrier to close approach of the vesicles.