The crystal structures of the simple monoclinic halogen-bonded complexes pyridine-ICl and pyridine-IBr have been redetermined using modern X-ray and neutron diffraction techniques. The representation quadric surfaces for the thermal expansion tensor have been found to consist of a hyperboloid of one sheet. Negative thermal expansion occurs parallel to the crystallographic b axis, whilst there is colossal thermal expansion in a direction approximately parallel to the a* direction. These effects arise because of a decrease in the strength of an already weak C-H center dot center dot center dot X (X = Br, Cl) hydrogen bond with increasing temperature. The decrease in strength of the hydrogen bonding originates because of the reduction in the strength of the halogen bond formed between the nitrogen atom and the IX moiety with increasing temperature. Thus since at 298 K the I-X bonds are shorter than at 110 K, there will be a smaller partial negative charge on the X halogen at 298 K, leading to a weaker C-H center dot center dot center dot X hydrogen bond.