A process is provided for the preparation of 9,10-dihydro-9-oxa-10-organylphosphaphenanthrene-10-oxide and derivatives of same substituted on the phenyl groups, in which: (a) 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOP) or a derivative of same is reacted in the presence of at least one mono- or polyhydric alcohol with at least one ortho ester with formation of a first intermediate product, (b) the intermediate product from step (a) is optionally reacted with at least one further mono- or polyhydric alcohol with formation of a further intermediate product and (c) the intermediate product from steps (a) or (b) is transformed by addition of catalytic quantities of alkylation agent into 9,10-dihydro-9-oxa-10-organylphosphaphenanthrene-10-oxide or a derivative of same substituted on the phenyl groups.
