Industrial wastewater often has a high salt content, due to specific factories and to the use of seawater in coastal areas. Once it has reached the activated sludge treatment, saline wastewater may cause biomass inhibition and dramatic loss in purification efficiency. In the present work, the inhibiting effect of saline influent was experimentally evaluated through respirometric tests on samples of the incoming wastewater. Activated sludge was grown in bench-scale continuous flow stirred tank reactor (CFSTR), supplied with synthetic wastewater. Synthetic saline wastewater was prepared using sodium chloride and sodium sulphate at different concentrations. Samples of mixed liquor were drawn from the CFSTR reactor and fed with saline wastewater in shock-load or in continuous mode, then respiration activity and carbon removal yield were evaluated. In batch mode (shock-load), salt/biomass ratios between 0.37 and 30.7 gsalt gVSS−1 gave respiration inhibition between 4% and 84% respectively. The respirometry bioassay required a short response time and the values of respiration inhibition predicted the loss in carbon removal efficiency with a good accuracy. In continuous mode, for salt/biomass ratio of 35.5 gsalt gVSS−1, respiration inhibition of 81% was obtained. This work showed that a relation between inhibition in shock-load and inhibition in continuous mode can be determined, therefore, the procedure herein proposed allows performing off-line bioassays on real biological plants, to assess in real time the inhibiting effect of the incoming wastewater before it reaches the activated sludge reactor.