The problem of mass transfer into a rigid, spherical, liquid drop, accompanied by an instantaneous reaction between the absorbing solute and a diffusing nonvolatile reactant, within the drop, has been investigated. Experiments on a gas-liquid system were carried out with drops suspended from a buret where an acid-base reaction using phenolphthalein as an indicator was utilized to track the moving reaction boundary. A model, for the motion of this reaction front, using a quasi steady state approximation based on the notion of an "undisturbed" core, and inspired by the shrinking core model, has been proposed. The experimentally determined position of the reaction front shows a fair degree of agreement with the theory, and some deviation is observed. The observed time for conversion of the complete drop agrees reasonably well with the theoretical predictions. A comparison of the proposed model with the unsteady state, numerical solutions shows that the proposed model is a reasonable analytical approximation for certain combinations of the concentration and diffusivity ratios of the two reacting species.