The following characters are not allowed in values for the PartitionKey and RowKey properties: The forward slash (/) character The backslash (\) character The number sign (#) character The question mark (?) character http://blogs.msdn.com/b/windowsazurestorage/archive/2012/05/28/partitionkey-or-rowkey-containing-the-percent-character-causes-some-windows-azure-tables-apis-to-fail.aspx Also the % character while allowed by Azure presents problems on retrieval due to double decoding by Azure.