With the help of Bartell's equation the difference in resonance energy of the metastable intermediate states leading to substitutions (electrophilic) in positions 8 and 6 of 7-hydroxycoumarin, in positions 5 and 7 of 6-hydroxycoumarin and in positions 6 and 8 of 5-hydroxycoumarin have been calculated. On this basis it has been concluded that the position 8 should be more reactive than 6 in 7-hydroxycoumarin, position 5 should be more reactive than 7 in 6-hydroxycoumarin and positions 6 and 8 should have equal reactivity in 5-hydroxycoumarin in electrophilic substitution. These deductions have been compared with the experimental observations made in the literature.