N-(2-Hydroxypropyl)methacrylamide (HPMA) co-polymers containing disulfide and carbonyl thiazolidine-2-thione (TT) reactive groups in their side-chains (pHPMA-TT) were used as reductively removable chemical modification of the surface of cowpea mosaic viruses (CPMV). CPMV was used in this study as a model particle for viral gene delivery. The co-polymer reaction with CPMV surfaces carried out in aqueous solution was evaluated by monitoring the changes in the weight-average molecular weight and hydrodynamic size of the viruses using light scattering methods. The reaction conditions were optimized. The surface modification of CPMV with pHPMA-TT under selected conditions (concentrations of a coating polymer (c(p)) and NaCl) has not influenced the size distribution of the viral particles. The uncharged polymers bound to the viral surface via biodegradable S-S bonds can be fully removed by the exchange reaction with reductive dithiothreitol. To achieve optimal covering of viral surfaces, the positively charged reactive polymers (with or without biodegradable S-S bonds) should be applied at low concentrations (c(p)=0.1-0.5 mg/ml) and in presence of NaCl (0.15 M).