The diagenesis of ancient bone collagen appears bewildering in its variety. We have simulated collagen diagenesis using a simple conceptual model in which the degradation is controlled by two key processes: (i) depolymerization of collagen by chemical hydrolysis of peptide bonds; and (ii) dissolution (i.e. melting free) of those polypeptide fragments retained by fewer than a critical number of hydrogen bonds, biodeterioration being ignored. The model predicts that the curve describing weight loss is approximately sigmoidal in form, a pattern in accord with the sparse experimental data for non-mineralized collagen. The conclusion from studies of mineralized collagen is that the weight loss curve is exponential (i.e. a first-order reaction), but we argue that the experimental design of those studies was flawed. Other predictions of the model relating to the changing size distribution of residual fragments, N-termini, physical properties and significance of cross-linking are reported. Cross-linking appears to be a particularly significant phenomenon, which causes a marked tailing of the weight loss curve and thus the enhanced preservation of cross-linked, low collagen bone. The validity of applying this simple conceptual model to archaeological collagen is discussed.