Several direct or indirect approaches have been proposed to measure diffusion coefficient of gases into liquids. The main complexity of indirect techniques such as pressure decay method is interpreting early pressure–time data which strongly affected by incubation period effect or convective instability. In the current approach

accurate apparatus and precise experimental setup including a high pressure and temperature PVT cell

a high precision Sanchez pump

heating and recording sub-system are implemented and a novel data analysis procedure is applied to modify pressure decay method. The effect of incubation period is reduced remarkably and diffusion coefficient of carbon dioxide in water in wide range of pressures and temperatures is determined and the effects of temperature

pressure and carbon dioxide phase alteration from gas to supercritical are investigated and the value of uncertainty is estimated. Furthermore

diffusion coefficient of CO

2

and methane in an oil sample from one of the Iranian southwest oil formations is determined precisely using the experimental approach while no incubation period is detected. The results showed that incubation period dura

tion decreases with increasing diffusion coefficient. Additionally

when CO

2

state is gas

rate of increasing diffusion coefficient with pressure is decreased with temperature and when CO

2

state is supercritical

the rate of increasing diffusion coefficient with pressure is decreased significantly.