During partial liquid ventilation (PLV) the knowledge of the quantity of exhaled perfluorocarbon (PFC) allows a continuous substitution of the PFC loss to achieve a constant PFC level in the lungs. The aim of our in vitro study was to determine the PFC loss in the mixed expired gas by an absorber and to investigate the effect of the evaporated PFC on ventilatory measurements. Method. To simulate the PFC loss during PLV, a heated flask was rinsed with a constant airflow of 4 L min^-1 and PFC was infused by different speeds (5, 10, 20 mL h^-1). An absorber filled with PFC selective zeolites was connected with the flask to measure the PFC in the gas. The evaporated PFC volume and the PFC concentration were determined from the weight gain of the absorber measured by an electronic scale. The PFC-dependent volume error of the CO2SMO plus neonatal pneumotachograph was measured by manual movements of a syringe with volumes of 10 and 28 mL with a rate of 30 min^-1. Results. Under steady state conditions there was a strong correlation (r ² = 0.999) between the infusion speed of PFC and the calculated PFC flow rate. The PFC flow rate was slightly underestimated by 4.3% (p < 0.01). However, this bias was independent from PFC infusion rate. The evaporated PFC volume was precisely measured with errors <1%. The volume error of the CO2SMO-Plus pneumotachograph increased with increasing PFC content for both tidal volumes (p < 0.01). However for PFC flow rates up to 20 mL/h the error of the measured tidal volumes was <5%. Conclusions. PFC selective zeolites can be used to quantify accurately the evaporated PFC volume during PLV. With increasing PFC concentrations in the exhaled air the measurement errors of ventilatory parameters have to be taken into account.