In this work we address the effects on the conductance of graphene nanoribbons (GNRs) oforganic molecules adsorbed at the ribbon edge. We studied the case of armchair andzigzag GNRs with quasi-one-dimensional side-attached molecules, such as linearpoly-aromatic hydrocarbons and poly(para-phenylene). These nanostructuresare described using a single-band tight-binding Hamiltonian and their electronicconductance and density of states are calculated within the Green’s function formalismbased on real-space renormalization techniques. We found that the conductanceexhibits an even–odd parity effect as a function of the length of the attachedmolecules. Furthermore, the corresponding energy spectrum of the molecules can beobtained as a series of Fano antiresonances in the conductance of the system. Thelatter result suggests that GNRs can be used as a spectrograph sensor device.