  We report a detailed spectroscopic investigation of temperature-induced
valence and structural instability of the mixed-stack organic charge-transfer
(CT) crystal 4,4'-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). DMTTF-CA is
a derivative of tetrathiafulvalene-chloranil (TTF-CA), the first CT crystal
exhibiting the neutral-ionic transition by lowering temperature. We confirm
that DMTTF-CA undergoes a continuous variation of the ionicity on going from
room temperature down to $\sim$ 20 K, but remains on the neutral side
throughout. The stack dimerization and cell doubling, occurring at 65 K, appear
to be the driving forces of the transition and of the valence instability. In a
small temperature interval just below the phase transition we detect the
coexistence of molecular species with slightly different ionicities. The
Peierls mode(s) precursors of the stack dimerization are identified.
