An accessory for carrying out ATR measurements in a spectrometer such as an FT-IR spectrometer. The accessory has input optical elements for directing a beam of infra-red radiation. to an HATR crystal 75 and output optical elements for directing radiation leaving the crystal towards an IR detector. The optical elements include a pivotable element which is adjustable automatically to a position in which it causes the radiation to be incident on the crystal at an angle of incidence appropriate for that crystal.