Something like this should do it for you. Change the sheet name and cell
reference as needed.
It will look at the specified cell and if nothing is in it, it will present
the message and cancel the save operation. If something is in the cell, it
just saves as usual.

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
If IsEmpty(Worksheets("Sheet1").Range("F9")) Then
MsgBox "You must make an entry on Sheet1, in Cell F9 before
saving.", _
vbOKOnly, "Save Cancelled"
Cancel = True
Exit Sub
End If
End Sub

This page will help you in getting the code into the right place in the
workbook:

http://www.jlathamsite.com/Teach/WorkbookCode.htm

"AucklandAssault" wrote:

I'm trying to stop users from saving a file unless they have entered an
explanatory note in a specified cell (or perhaps range). Is this possible
and, if so, how?

Instructions for a warning message would also be useful.