 His spell with Trabzonspor included two loan spells, the first to Akçaabat Sebatspor (13 apps in all competitions) and the second to Kayserispor (3 apps in all competitions).The latter became Sönmez's fourth permanent career club as he joined Kayserispor permanently ahead of the 2007–08 season.He made 10 appearances over two competitions for the team before leaving at the end of the season to sign for Samsunspor, his parting gift from Kayserispor was the 2007–08 Turkish Cup which him and the club won in a final against Sönmez's old club, Gençlerbirliği.