Modeling the diffusive mass transfer in various processes such as oil recovery, pharmaceutical, food, and chemical industries requires the accurate measurement of the molecular diffusion coefficient. In heavy oil and bitumen recovery, the injected gaseous solvent dissolves across the gas-oil interface and aids in higher oil recovery. On the other hand, a pressure drop in heavy oil causes the exsolution (degassing) of the light hydrocarbon from the heavy oil phase, whereby the supersaturation promotes the oil recovery. Hence, the exsolution and dissolution diffusion coefficients are the vital parameters used in the design and simulation of the bitumen and heavy oil recovery. Despite the significance of the exsolution and dissolution kinetics in bitumen recovery, relevant experimental data is lacking in the literature. While the dissolution kinetics of the gas-liquid systems are studied to some extent, there is an evident gap in the quantity and quality of the exsolution diffusivity data in the literature. In this study, a novel experimental setup was developed and validated for conducting the exsolution and dissolution experiments of the gas-liquid systems. The developed experimental setup allows performing a pair of constant-pressure exsolution/dissolution experiments, gas-liquid saturation (equilibration), and liquid phase density measurement. An analytical model was adapted for analyzing the experimental data. The model uses the measured rates of gas exsolution and dissolution to estimate the exsolution and dissolution diffusion coefficients, respectively. The developed methodology was used to report the first datasets on the exsolution and dissolution diffusivity and kinetics of the ethane/n-heptane and methane/bitumen systems. The results indicated that the gas exsolution is a faster process compared to the gas dissolution. Analysis of the experimental data using the Arrhenius equation revealed lower activation energy for exsolution compared to dissolution. The molecular diffusion coefficients of gas exsolution in ethane/n-heptane and methane/bitumen were found in the range of (2.02-5.44)×10-6 m2/s and (1.13-14.2)×10-8 m2/s, respectively. Also, the molecular diffusion coefficients of gas dissolution in ethane/n-heptane and methane/bitumen were found in the range of (1.76-11.80)×10-8 m2/s and (1.95-56.9)×10-9 m2/s, respectively.