We are trying to generate MS Excel workbook using OOXML and populate data using SSIS. We are able to generate Workbook and sheets, also able to create columns and insert data in Header cell. We can also populate data using SSIS. But the Sheet (DocumentFormat.OpenXml.Spreadsheet.Sheet) and all cells (DocumentFormat.OpenXml.Spreadsheet.Cell) becomes OpenXmlUnknownElement. So we are not able to read sheet / cell using following Code. Sheet sheet = workbookPart.Workbook.Descendants().Where(s => s.Name == "Sheet1").SingleOrDefault(); We are able to read the same file if we first open it using MS Excel and save.