ZSM-5 samples with a silicon-to-aluminium atomic ratio of 25 were impregnated with gallium, indium or thallium salts. The acidic and textural properties of the samples were determined either before or after reduction. The acidity was investigated by infrared spectroscopy using pyridine (Py) as test molecule, titration with KOH or Py as well as ammonia adsorption. Differences were revealed concerning the dispersion of impregnated species and the distribution of the acidic sites. The sites containing ionic thallium were spectroscopically identified; the other sites were indirectly evidenced. Under our impregnation conditions, gallium species seem to enter inside the zeolite to a higher extent than was reported for similar samples. The contribution of indium to the sample acidity was reduced because it is deposited as a distinct phase onto the external surface. An adsorption mechanism operating during impregnation was proposed on the basis of electrochemical measurements; it was used to account for the acidity data. The textural properties were interpreted in terms of the deposition of the impregnation species near the pore mouth, hindering the entrance of the test molecules.