Place cucumbers and red onion in a heatproof serving bowl.
In a small nonreactive saucepan, combine sugar, vinegar, cloves, salt, pepper and 1/3 cup water. Bring to a boil over medium heat, stirring to dissolve sugar. Reduce to low, cover and cook 5 minutes.
Pour vinegar mixture over cucumbers and onions. Stir to mix. Cover and refrigerate at least 2 hours before serving