Phosphorene, the two-dimensional counterpart of black phosphorus, is under current intense investigation in order to be applied in gas sensor devices. In regard to the material excited-state properties, these may be sensitive to molecular adsorption. Therefore, in this work we theoretically study the change in the optical properties of phosphorene-CrO3 systems considering different CrO3 surface coverage (0.0%, 34.3% and 68.6%). The CrO3 molecule is a powerful oxidizer and a suspected carcinogen. To determine rigorously the optical properties of CrO3 adsorbed on a surface is mandatory the use of the G(0)W(0) approximation and the solution of the Bethe-Salpeter equation (BSE) with the inclusion of van der Waals forces. As part of the results, this work shows the electrical band gap values obtained by the application of the G(0)W(0) approximation, optical band gap values derived from the solution of the BSE and an analysis on the optical in-plane anisotropy of the composed phosphorene-CrO3 systems. Ultimately, results show that the band gap, the optical absorption spectrum and the optical in-plane anisotropy of phosphorene can be broadly tuned by changing the amount of CrO3 surface coverage and molecular disposition. (C) 2017 Elsevier B.V. All rights reserved.