INTRODUCTION: The use of fingermarks as evidence is ubiquitous to forensic science, having been utilised for identification in some form or another for well over a hundred years. Chemical methods for visualising fingermarks typically function by forming a visible dye or fluorophore, although more recently this idea has been expanded to include ultraviolet and near-infrared emitting species. The Home Office Fingermark Visualisation Manual lists at least seven Category A processes that rely on the chemical transformation of residues present within fingermarks, or affinity of those residues towards certain chemical dyes. The affinity of iodine towards fingermarks is long known, and it features in two Category B processes described in the Fingermark Visualisation Manual: iodine fuming and iodine solution. The underlying cause of this affinity has been debated, with the polarizability of iodine in the presence of Lewis bases thought to be the driving force behind the interaction. Transient, reversible interactions between iodine and the alkene functional groups of squalene constitute one of the likeliest candidates in the context of fingermark residues. Chemical fixing of iodine to overcome the drawbacks of transient development and poor contrast provided by iodine alone has been achieved several ways, the most common being through its encapsulation by starch, or by reaction with α-naphthoflavone (a.k.a. 7,8-benzoflavone). The chromophore from the latter fixing reaction has been previously reported as a gem-iodo species formed after substitution of the ketone group by iodine (vide infra), though this unpublished report provided no supporting instrumental evidence for that assignment.