Molecular diffusion is greatly enhanced when it takes place in a carrier that is moving in a reciprocating fashion within a pipe. The enhancement reaches three to four orders in gases if the oscillation frequency and amplitude are tuned to the physical properties of the diffusing gases and of the carrier gas. Unexpectedly, the heavier gas shows a greater enhancement than the lighter one and therefore diffuses faster. The enrichment of the lighter gas, which we describe in this paper, is therefore more complex than the enrichment of the heavier one. It can only be achieved productively if the standard diffusion protocol is modified and includes diffusion in a continuous, upstream flowing carrier and if the carrier is condensed in the container that receives the feed. Experimental results confirm the theory. This theory predicts unusual findings : thus, separation ratio and yield may be freely set by adjusting the numerical value of independent variables such as the rate of inflow of the feed. Moreover, the yield is found to rise proportionately with the separation ratio! The technique may therefore be practically useful in addition of being quite interesting. Cost estimates for the purification of He are given.