Ultrasonic velocity profiling with pressure difference (UVP-PD) was demonstrated to be a successful, non-invasive, in-line measurement system for instantaneous velocity and rheological flow profiling of complex, opaque fat blends. Model systems of 25% Akomic, 75% rapeseed oil; and 25% Akomic, 74% rapeseed oil and 1% Grindsted® Crystalliser 110 were compared under real process conditions with UVP-PD. Results indicated that the sample containing the crystalliser had twice the viscosity of the control. These in-line results are in agreement with previous off-line results, and offer the chance to probe the mechanics of fat blend physics under real, dynamic conditions. © 2008 Institute of Food Science and Technology.