A theoretical model for zero-length column (ZLC) desorption curves for biporous adsorbent particles is presented and solved to yield an analytical expression for the transient desorption curve. The solution is shown to reduce to the simpler limiting form for a single resistance system under conditions such that either the micropore or macropore resistance is dominant. The conditions under which the single resistance approximation is appropriate are discussed in relation to the previously reported behaviour of representative systems (nitrogen and oxygen in carbon molecular sieve and 5 A zeolite pellets).