In small saucepan, bring balsamic vinegar to a boil over medium-low heat. Cook 10 to 20 minutes, or until balsamic has reduced to a thicker glaze. Remove and transfer to bowl, allow to cool.
Arrange tomatoes, mozzarella slices, and basil leaves on a platter. Drizzle olive oil over the top, getting a little bit on each slice. Repeat drizzle with the balsamic reduction.