The work presented in this book consists of synthesis of naturally and non-naturally occurring isocoumarins and kinetic studies for the hydrolysis of isocoumarin and ketoesters. The isocoumarin, 3-n-propyl-6,7-(methylenedioxy)-1H-2-benzopyran-1- one, Xyridin A, was isolated in 1995. It has been used as folklore medicine in many countries. Its total synthesis has been carried out successfully. Another natural product, 6, 8-dihydroxy-3-[(2''-ethyl-(4''''-methoxyphenyl)]-3,4- dihydro- isocoumarin, (dl)-Agrimonolide, was isolated in 1958. Partial synthesis of this isocoumarin has been carried out successfully which may be a convenient route for its total synthesis. Non-naturally occurring isocoumarins and their dihydro derivatives have been synthesized as well. They include a series of methoxyphenylisocoumarins and dihydroisocoumarins and halogenated isocoumarins and dihydroisocoumarins. In the kinetic studies, the hydrolysis of 3-(2''''-chloro-6''''-flurobenzyl)isocoumarin and methyl-2[2''-oxo-3''-(2"-chloro-6"-flouro- phenyl)propyl]benzoate with KOH solution were monitored to probe the mechanism of the reaction. Role of solvent in the reactions have also been investigated.