Intensity Modulated Radiotherapy (IMRT) is extensively used for cancer patients treatment in modern Radiotherapy Clinics. Patient-related Quality Control (QC) is essential part of IMRT clinical implementation, as fluencies of Intensity Modulated (IM) fields has to be independently verified prior to delivery of the IMRT plan to the patient. In most cases, combination or ionometric and film dosimetry is in use for patient-related QC. The aim of this study was to evaluate volume-averaging effect of different ionization chambers used for ionometric dosimetry of the IMRT fields. Examination of the response of three ionization chambers of different sensitive volumes was performed, using set of static and IM fields. Chambers were cross-calibrated in static field of size, sufficient to cover with homogeneous dose chamber with the largest sensitive volume. Dynamic IM fields with different dose gradients through chamber sensitive volume were designed. Fluencies of all IM test fields were verified by means of the film dosimetry. Measurements in the penumbra region of the static field were used to separate effects of dynamic dose delivery from the chamber averaging effect. Penumbra profile of the static field was acquired using all detectors under investigation. Differences in the chambers response larger than 8% were found for measurements in high dose gradients both in penumbra of static field, and in gradient region of the dynamic IM field. Chamber of the largest volume had worst results in penumbra region of the static field and largest deviation from analytically reconstructed penumbra of IM fields.