[Mobiu5]

From what I understand, an ester will form but I'm confused how the hydronium ion will begin to break apart the lactone. Will it attack the carbonyl group first or use the second oxygen to begin the reaction?

[camptzak]

http://www.chemtube3d.com/model/nucleophilic_substitution/acid-cat_ester_hydrol/mechanism.png

here is an ester hydrolysis mechanism, it works in the same fashion. The carboxylic acid and and the alcohol are in equilibrium with the ester and H₂O.
I am pretty sure that only unstable lactones like butyrolactone are susceptible to acid hydrolysis. I think most of the other ones will not react with H₃O⁺.