Flow disturbances associated with the physical presence and sink action of two types of engine-cylinder-gas sampling probes were investigated in a steady-flow rig. Flow visualization and comparisons of concentration profiles obtained with an isokinetic probe and the two engine-type probes showed that the major flow disturbance was the creation of a flow around the probe tip. This flow is directed along the leading edge of the probe, parallel to the probe axis, and in the direction of the sampling orifice such that the apparent origin of a sample is some distance, ~0, behind the actual location of the sampling orifice. The influences of seven sampling parameters on ~O were determined. Probe diameter, sampling rate, probe angle with respect to the free stream, and probe insertion length all had a significant effect on the magnitude of ~0-The shape of the probe tip and the orifice location eccentricity had a lesser effect, while probe Reynolds number only weakly affected the ratio ~o/D. The probe diameter, D, also affected the measured concentration by altering the diffusion processes. When sampling in low velocity streams, an intermittent sampling probe produces less concentration profile smearing than a steady-flow probe because the sampled gas composition is not averaged over as large an area. A criterion based on a quasi-steady sampling model is given for determining engine conditions and sampling parameters under which intermittent sampling produces essentially the same probe displacement and concentration measurements as obtained for steady flow.