 In retirement he focused on Mon–Khmer languages; as a guest researcher at Mahidol University he did extensive fieldwork in Thailand and Laos, particularly on the Mlabri tribal language, an endangered and previously undescribed dialect of a Khmuic language.His 1995 book described Mlabri phonology, morphology and syntax whilst supplying a lexicon with illustrative examples.In 1978 he was elected a member of the Royal Danish Academy of Sciences and Letters.