Hi p45cal, thanks for your reply, it helps me improve to see better and cleaner ways to write the code. Unfortunatelly it keeps returning T+1 on the ISINs that don't appear on book2. How is it possible that tehe code shows T+1 when the value is not even on book2, it doesn't make sense to me and i don't understand it.
On other columns of the book1 i have some code write in with a simple vlookup function and in these cases if the value doesn't appear on book2 it skips the cell and leaves the value that was already in it. To give you and example I'll show you a part of the code where if the ISIN it's not found on book2 it leaves the value that was in the cell untouched (This works completely find). Thanks in advance again!

'Cut Off


i = 2


Do While Cells(i, 1).Value <> ""


 Cells(i, 11) = Application.WorksheetFunction.VLookup(book1.Sheets("Hoja1").Cells(i, 1), book2.Sheets("Hoja3").Range("A:Q"), 17, 0)
    i = i + 1
 
 On Error Resume Next


Loop