The ‘dimethylene tartrates’ are shown to be bidioxolan-4-yl-5,5′-diones. Both cis- and trans-2-trichloromethyl-5-phenyldioxolan-4-ones have been isolated. N.m.r. assignments have been made for these and for other dioxolan-4-ones, and it is shown that long-range coupling between ring protons in this series occurs for trans- as well as for cis-stereoisomers.