A process is described for the preparation of 3-isochromanone having the general formula (I): the process comprising reacting ο-tolylacetic acid with a suitable halogenating agent to form a compound of general formula (III): wherein R is halo, reacting the compound of general formula (III) with a suitable halogenating agent to form a compound of the general formula (II): wherein X is halo; and (a) treating a compound of the general formula (II): with an aqueous base to a pH of 7 to 14 followed by acidification; or (b) adding the compound of the general formula (II) to water with the simultaneous addition of base to maintain the pH between 5 and 8; or (c) treating the compound of the general formula (II)with water or aqueous acid followed by pH adjustment to a pH between 5 and 8. Typically both R and X are chloro. 3-Isochromanone is useful, inter alia, as an intermediate in the manufacture of agricultural products, especially fungicides of the strobilurin type.