Laser cleaning studies were performed on samples of ancient Pentelic marble covered with black encrustations by employing a Q-switched Nd:YAG laser. Elemental analysis by laser induced breakdown spectroscopy (LIES) has been applied as a real-time diagnostic technique for the cleaning process, with the aim of safeguarding against potential damage. In addition to the elemental composition, this technique can also provide information on the depth and morphology of the crust when combined with ablation rate studies. In this way the choice of the optimum laser parameters for effective cleaning is facilitated. The analytical and structural composition obtained by LIES compares well with the results obtained by traditional analytical techniques, such as scanning electron microscopy, energy dispersive X-ray analysis, Fourier transform infrared spectroscopy and optical microscopy. This indicates that LIES can be used as an autonomous on-line diagnostic technique, providing information necessary for accomplishing the efficient laser cleaning of marble artefacts.