Fibre Optics Reflectance Spectroscopy ( FORS ) is a well-established technique, widely used in the conservation field for in situ investigations and non-invasive diagnostics on traditional artworks. Nevertheless, there is little knowledge on the use of this technique for investigating modenr artists materials used in contemporany artworks. The present study, carried out within the wider framework of the EC funded Project "POPART" ( Preservation of Plasic ARTefacts in museum collections" ), was aimed at investigating the possibility of extending the applications of FORS to synthetic polymers, and proposing this technique as new non-invasive analytical tool for diagnostics on plastic artworks in museum collections. Thus, a real case was selected from the permanent collection of the Contemporany Art Centre " Luigi Pecci " ( Prato, Italy ). The artwork, created in 1900 by the artist Stefano Arienti, was entirely constituded of expanded polystyrene ( EPS ) and it showed diffused signs of degradation. The need of singling out the main causes of deterioration in order to prevent further damages was evident. An in situ FORS measurements campaing was performed with the aim of gaining in-deph information about its conservation state and identifying the main agents responsible for the observed degradation. This emphasized the importance of an indterdisciplinary approach to the solution of conservative problem in the field of contemporany art.