In a recent paper (Goulletquer & Wolowicz, 1989), data are presented which show that determination of the organic content of mollusc shells by weight loss on ignition may over-estimate this component by a factor of 2·5 to 4·8. They then go on to conclude in their discussion that Rodhouse et al. (1984) had over-estimated the organic content of Mytilus edulis shells by a factor of 2·5 through combustion at 540°C.