because you have / in the string, consider to encode the string first, for examplestring a = "WeTe6zmUBA+T4g/mJH1ZN0G+pAnaCP3l42DDE7nHIQf/9EOLA1m8sQOIPhbUFK1VKOGsS6fBh0EwmHIaUdyaw6wcBiArudI5GPkJlw2ORs9WkvaUg6q4X6ZVTEaNjihFHJwl/F46q3Jx8Czy4WZdHCrh62urzL4rQPaLdsEhOxI=";Response.Redirect("TestBase64.aspx?x=" +  HttpUtility.UrlEncode(a));