In a 2- to 3-quart pan over high heat, bring 1 1/4 cups water to a boil. Stir in couscous and basil. Remove from heat, cover, and let stand 5 minutes.
Meanwhile, in a small bowl, whisk together olive oil, vinegar, and garlic. Fluff couscous with a fork and stir in peppers and olives. Add oil mixture and mix well.