The German Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area has evaluated di‐n‐butyl phosphonate and di‐n‐octyl phosphonate to derive a maximum concentration at the workplace (MAK value), considering all toxicity endpoints. Unpublished study reports and publications are described in detail. In vitro genotoxicity tests are negative; in vivo data are not available. Critical effect for both substances is irritation of skin or eyes. No repeated dose studies are available to define possible local and systemic effects. Therefore, no MAK value can be established and di‐n‐butyl phosphonate and di‐n‐octyl phosphonate are assigned to Section II b of the List of MAK and BAT values. Skin contact is not expected to contribute significantly to systemic toxicity. A maximization test with di‐n‐octyl phosphonate and an in vitro test with di‐n‐butyl phosphonate do not show sensitizing activity.