The observation that magnesium compounds, unlike calcium carbonate, produce on neutral, pure cellulose paper opposite effects in the course of accelerated ageing, according to whether this is carried out in a dry or moist environment, is discussed in terms of significance of testing methods. Whereas the other procedures examined are acceptable from a chemical standpoint, it is felt that deacidification with magnesium compounds should be discontinued until further investigation sheds more light on its implications. -- AATA