Aspiration dust probes of some isokinctic samplers have such a construction that enables them to measure the gas velocity at the point they arc placed in a duct, the measurement being required for maintaining isokinctic conditions and in determining the dust mass flow rate in the duct. The gas velocity is correlated with a probe specific pressure difference, a quantity measured directly when the train is in operation. This relationship is a mclrological characteristic of a given probe, being established in a calibration procedure. Two types of the above-mentioned probes, namely a pressure balance-type and an in-stack filtration probes (in two versions) combined with a type S velocity sensor, have been tested. The behaviour of the characteristics of the probes was studied in the flows of different turbulence at a laboratory stand. The achieved results, presented in the paper, show that the turbulence state of gas flow is the factor that shapes the mctrological characteristics. Facing the fact that the turbulence of gas flow in industrial installations is not identified during the dust concentration gravimetric measurements, this parameter constitutes a source of uncertainty of the gas velocity measurement. For the probes under test, the values of this uncertainty, varying with the gas velocity, were determined. The test and calculation procedures were described.