Jonny Gomes hit three homers and drove in five runs to give Arroyo plenty of support.
Arroyo (11-11) made the 218th start of his career, the last four years of which have been spent with the Reds. Two of his shutouts have come this season and this was his ninth complete game in the majors.
He allowed a leadoff single to Adam Dunn in the second and a single to Ryan Zimmerman to open the seventh. Arroyo struck out three, walked one and threw 102 pitches.
Gomes hit a two-run homer in the second, added a two-run shot in the fourth and then hit a solo blast in the sixth.
Zimmerman's single extended his hitting streak to 16 games but a 17-game streak for Cristian Guzman came to an end when he went 0-for-4.
After winning eight straight games, Washington has dropped three in a row.
Copyright 2009 by United Press International.