The aim of this study was to investigate the effect of formulation on the pharmacokinetics of diclofenac in two sustained release formulations (formulation A and Voltaren SR) after oral delivery. The dissolution of diclofenac from sustained release formulation was pH-dependent. While drug released from both formulations increased with increased pH, the release kinetics of these two formulations was different. The pharmacokinetic study was conducted in 12 healthy subjects administered with multiple doses of 100mg of diclofenac in a crossover design. There was a significant difference in area under the plasma concentration-time curve [AUC(0-24)] and C(max) observed. The formulation with a reduced diffusion exponent with increased kinetic constant results in increased absorption of diclofenac in vivo. This study demonstrated the impact of release mechanism of the formulation on the absorption in vivo.