Modern high yield pulps, typified by chemithermomechanical pulp (CTMP), are likely to be used increasingly for printing, writing and publication grades where longevity is a major consideration. This paper reports an investigation of the ageing of papers containing CTMP, and sized both with rosin/alum and alkyl ketene dimer(AKD) systems, using paper made specially for the study on a pilot papermaking machine. Emphasis was placed on mechanical properties, in view of the concern of librarians and archivists with the embrittlement of paper due to acid hydrolysis of the cellulose. The results show that use of CTMP has a negligible effect on rates of loss of strength; in a series of papers containing various proportions of CTMP with a bleached softwood sulphate pulp, sized with AKD, the rate of degradation was very similar, and acceptably low. Naturally, the papers containing CTMP are not initially as strong, and will become yellow on exposure to light; in judging whether or not such papers are acceptable for long term use, all these factors need to be considered with respect to particular applications.