A canal surface viscometer has been designed and constructed to measure the in‐plane steady shear viscosity (&eegr;s) of monolayers at the air/water interface. The technique involves forcing a monolayer to flow through a canal to another monolayer under a constant surface pressure gradient and measuring the area flow rate. The instrument incorporates a feedback electronic system to maintain the surface pressure on each side of the canal to about 0.1 dyn/cm during monolayer flow. The canal design is modified from that of an earlier design in the literature to eliminate artifacts that give rise to systematic variabilities as reported. Specifically, it is demonstrated that the glass‐paraffin canal design used in the past produces ill reproducible results in contrast to an all Teflon canal, which gives rise to consistently reproducible surface viscosity data. A comparison is made between &eegr;svalues obtained from the canal viscometer and the knife‐edge ring surface viscometer, another technique reported in the literature. The comparison reveals the importance of controlling key experimental variables which can have an order‐of‐magnitude effect on the surface viscosity data.