Thank you all for your answers. The problem was in trailing blanks, as you
suggested; my admin tool doesn't showed me the blanks at the end of the
varchar fields. But, It remained unaswered why did those blanks get there.
It seems to be a problem with IBX: I used a TIBQuery with a insert
statement:

insert into DATOSTERMINALES(IDDATOTERMINAL,CANTIDAD, CODIGO, CONTRATO,
FECHARECEPCION, GRUPO, LOTE, NROTERMINAL)
values
(GEN_ID(GENRECIBIDOS,1), :CANTIDAD, :CODIGO, :CONTRATO, :FECHARECEPCION,
:GRUPO,
:LOTE, :NROTERMINAL)

The values assigned to the params where ok, all of intended length. But I
still got those trailing blanks in the bd. I didn't use a trace tool because
i didn't have one, so i don't know what was really sent to the server.
The solution: I changed the TIBQuery components for TIBDataset, let the
components generate their own sentences, and the problem is gone.

I went through all of this detail just in case it is useful to anyone else;
the conclusion is that it is not a Firebird's bug but an IBX (v7.04/08)
one...

Thanks again

Ernesto Cullen