In order to achieve synergic environmental effect CO₂-EOR can be applied to improve oil production and reduce carbon dioxide emission. Greater results can be obtained by the CO₂ huff-n-puff process which in the case of oil reservoirs is based mainly on the oil swelling effect. During the soaking period the primary mechanism distributing CO₂ in the reservoir is molecular diffusion. To calculate mass transferred to oil phase molecular diffusion coefficient is used. Its determination can be based on direct or indirect techniques but no standard method is normed. This paper presents a pulse-echo pressure-volume decay technique to determine oil-CO₂ diffusion coefficient under reservoir conditions with the use of acoustically monitored separator. In the proposed method measurements of difference between signal routing are recalculated to volume change in order to compute diffusion coefficient. Experimental procedure was conducted for three different temperatures. The obtained results are consistent with previous studies on diffusion coefficient determination. The proposed technique is more reliable and yields straightforward solution due to simultaneous binary measurements of pressure and volume system.