Polymer compositions, for example of polyethyleneterephthalate bottles or preforms, include a reheat additive which has reducing transmission/increasing absorbance across the IR region as the wavelength increases. The reheat additive may be a titanium nitride, made by a plasma vapor deposition technique. Advantageously, the material may be used at a lower level than hitherto known materials or may be used at the same levels as hitherto but provide a greater reheat effect. Example 3a shown in the figure illustrates the absorbance of a preferred material.