Theory and simulation of the dynamical phenomena that characterizes gas diffusion in a cholesteric film are presented. A classical mass transfer theory for cholesteric liquid crystals was used to construct a model that describes the one dimensional transient gas diffusion in a film. The boundary conditions that describe the concentration and orientation conditions in a gas-liquid crystal surface were obtained using the Euler-Lagrange equations for surface reorientations. Numerical solutions to the coupled mass transfer-orientation equations are presented and used to develop a comprehensive view of the phenomena. The main governing parameters that control gas diffusion in a cholesteric film are identified. Two different regimes are identified : (i) diffusion limited regime, and (ii) orientation limited regime. The diffusion limited regime is characterized by strong concentration-orientation couplings, enhanced mass transfer, and up-hill diffusion. The orientation limited regime is characterized by weaker concentration-orientation couplings, and weaker mass transfer enhancements. Conditions that lead to enhanced gas absorption are identified, characterized, and explained in terms of the orientational contribution to the mass flux. Conditions that lead to the uncoiling of the cholesteric helix into a nematic ordering are identified, and the kinetics of the phase transformation is characterized.