Hi,
As Ben Chase wrote before the problem happens because as default BindingProperty set to "Text", but for correct work it must be set to "EditValue"
Here is a code what made good percent textbox ready
' DataSource Part
Me.textbox.BindingField = "manufacturer_mwst"
' BindingProperty is important!
Me.textbox.BindingProperty = "EditValue"
Me.textbox.BusinessObject = Me.Bbay_manufacturerBO1
Me.textbox.BusinessObjectEvaluated = True
' Mask Part
Me.textbox.Properties.Appearance.TextOptions.HAlignment = DevExpress.Utils.HorzAlignment.Far
Me.textbox.Properties.Mask.EditMask = "P0"
Me.textbox.Properties.Mask.MaskType = DevExpress.XtraEditors.Mask.MaskType.Numeric
Me.textbox.Properties.Mask.UseMaskAsDisplayFormat = True
Denis