Hypothesis: The functionality of magnetic nanoparticles (MNPs) relies heavily on their surface coating, which in turn affects the interactions between MNPs, and the formation of single-core particles or multi-core clusters. In this study we assessed the use of AC susceptibility (ACS) as a magnetic probe of the kinetics of coating and agglomeration of functionalised nanoparticles. We demonstrate the precision and sensitivity of ACS measurements to small changes in MNP coating using arginine-glycine-aspartic acid (RGD) tripeptide binding, and subsequently discuss how ACS can be used to optimise the preparation of polyethyleneimine (PEI) functionalised MNPs aimed at nanomagnetic transfection applications. Experiments: We varied the PEI loading of suspensions of MNPs exhibiting a combination of Brownian and Neel relaxation, and used dialysis to study the movement of excess PEI during the coating process. Numerical ACS simulations were employed to determine particle cluster sizes and polydispersity and the results compared with conventional dynamic light scattering (DLS) size measurements. Findings: ACS provided information on the MNP coating and agglomeration process that was not accessible through DLS due to the additional presence of non-magnetic polymer particulates in the suspensions. We consequently derived a simple method to obtain dense, uniform PEI coatings affording high stability suspensions without excessive quantities of unbound PEI. (C) 2018 Elsevier Inc. All rights reserved.