Action of acetic anhydride on 4-methoxyhomophthalic acid in the presence of pyridine at about 100° furnished 4-carboxy- and 4-acetyl-7-methoxy-3-methylisocoumarins and a small amount of a substance (VIa), bot at room temperature it mainly gave 4-acetyl-7-methoxyisochroman-1,3-dione. This isochromandione was converted quantitatively on t.reatment with 80% H2SO4. into 4-carboxy-7-methoxy3-methylisocoumarin getting partly decarboxylated during the reaction to 7-methoxy-3-methylisocoumarin depending on the temperature of the reaction. Various reactions on these isocoumarins and isochromandione have been studied with interesting results. 2-Carboxy-4-methoxybenzyl methyl ketone was obtained in excellent yield by the action of aq. NaOH on them. The ketone cyclodehydrated to 7 -methoxy-3- methylisocoumarin and on reduction with sodium borohydride furnished 7-methoxy-3-methyl-3,4-dihydroisocoumarin in very good yield. Similar results have been obtained by the action of acetic anhydride on homophthalic acid with pyridine, giving a new synthesis for 3-methylisocoumarin and 3-methyl-3,4- dihydroisocoumarin in good yields.