AbstractThis study investigates ad hoc knowledge in interpreting – knowledge specifically relevant to an interpreting task, usually acquired by interpreters at pre-service and in-service stages. A quasi-experiment asked eight English–Chinese professional interpreters to prepare for and interpret two speeches with different degrees of specialisation. Five datasets were collected: (1) screen and video recordings of their preparation processes; (2) their responses to a follow-up questionnaire; (3) their scores and answers on a domain knowledge test; (4) their interpreting performance, rated by both professional interpreters and domain experts; (5) the speaker's answers to a few open questions on domain knowledge. Parts of the first and second datasets were used to examine what ad hoc knowledge the interpreters needed; the remaining three datasets, together with recording data from the first, were used to investigate the extent of their need for this knowledge. The findings show that ad hoc knowledge is domain-relevant, yet its scope is highly interpreting-specific, being neither deep nor wide; and it is by nature different from domain knowledge, mainly including speech-relevant expressive knowledge about certain domain(s) and the contextual knowledge interpreters use in their preparation. The implications of these findings for interpreting practice and training are also discussed.