The "Input string was not in a correct format" messages comes from a standard System.FormatException raised by a call to int.Parse, added to the automatically generated assembly that does the deserialization. I don't think you can add some custom logic to that.

One solution is to do something like this:

    [XmlElement("IntField")]
    [Browsable(false)] // not displayed in grids
    [EditorBrowsable(EditorBrowsableState.Never)] // not displayed by intellisense
    public string IntFieldString
    {
        get
        {
            return DoSomeConvert(IntField);
        }
        set
        {
            IntField = DoSomeOtherConvert(value);
        }
    }

    [XmlIgnore]
    public int? IntField { get; set; }

It's not perfect, because you can still get access to the public IntFieldString, but at least, the "real" IntField property is used only programmatically, but not by the XmlSerializer (XmlIgnore), while the field that's holding the value back & forth is hidden from programmers (EditorBrowsable), grids (Browsable), etc... but not from the XmlSerializer.