This paper focusses specifically on the topic of slug velocity and the definitions and origins of the various velocities observed within horizontal slug flow pneumatic conveying. Testing was conducted and measurement of slug particle velocity and wave front and rear velocities confirmed existing observations in the literature that the slug wave travels faster than the individual particles. Examining the relationships between the measured velocities further, a model based on the conservation of mass in a slug provided a relationship between slug velocity terms, length, porosity and the stationary layers. For constant slug length conditions the new model was shown to be equivalent to the gas-liquid analogy model of Konrad [1]. The propagation velocity in the analogy of Konrad was shown to be the relative velocity term, which reflects the perceived change in velocity from the dynamic changes in slug length occurring due to the particle exchanges between the slug and stationary layers. Further-more, the porosity gradient between the slug and stationary layers was shown to impact both the velocity and layer fraction. The validation of the model showed very good agreement to the measurements and demonstrated that the slug particle and wave velocities should not be used interchangeably as is often done in the literature. (C) 2019 Elsevier B.V. All rights reserved.