Analysis of extended surface is limited to a great extent by the number of constraints or limiting assumptions that must be made in order to permit an analytic treatment to obtain workable design data. It is shown in this paper that one need not neglect the effect of bond resistance, at least when dealing with fins of rectangular profile. Equations are developed that permit an adjustment of fin efficiency and overall efficiency when dealing with plate-fin heat exchanger cores. The adjustment factors so developed can easily be incorporated into design procedures for exchanger elements that utilize adhesive bonding instead of the heretofore customary method of brazing the fin elements to the confining surfaces.