Hi Sarah, for a homogeneous population of binding sites the binding or association constant (Ka) = (kon / koff) - the on and off rates. The dissociation constant (Kd) is the inverse of this. So measuring Ka and the binding rate allows you to calculate koff. Bear in mind however that MIPs are not generally homogeneous binders. I hope this helps. Can anyone else help Sarah? Mike Whitcombe (SMI President)