A method was developed for measuring the time of flow in an Ubellohde viscometer to an accuracy of 3 ppm. For this purpose the following points were shown to be crucial: (a) use of a thermostat with a long time thermal stability of ±2×10−4°C; (b) closing the viscometer off to the outside during all stages of the experiment; (c) adoption of a standard procedure for cleaning and preparing the glass surface; (d) maintenance of the viscometer bulb and capillary always in contact with the fluid by raising level immediately after each experiment; (e) adjustment of the position of the viscometer reproducibly; (f) measurement of the time of flow to within 1 msec. A full description of all parts of the system except the thermostat is given. The thermostat has been described separately. Examples are given illustrating both the accuracy of the technique and the importance of each and all of the above points.