A key unit operation in most recycled paper mills is the flotation deinking process. In an ideal flotation cell, the froth (rejects) should only contain ink and other hydrophobic contaminants. In actual flotation cells, however, the rejects also contain small amounts of useful cellulosic pulp material. These pulp losses are a waste of raw material. The objectives of this thesis were to identify the dominant mechanism(s) responsible for pulp loss in flotation systems, and to attempt to quantify these mechanisms. Three potential causes of pulp loss were investigated: pulp flotation by bubble adhesion, pulp flotation by bubbles mechanically-trapped within fibre networks, and the transport of pulp fibres and pulp fines into the froth by hydraulic entrainment. Direct observation of bubble-fibre interactions in both bubble generation tests and in flow visualization studies revealed that bubbles are unlikely to adhere to wetted pulp fibres. The flow visualization studies also demonstrated that while bubbles can become trapped in fibre networks, these bubbles are easily released under flowing conditions. Mechanical entrapment is considered to cause significant pulp flotation only under quiescent conditions where pulp flocs were present. Based on results obtained from batch flotation experiments, it was determined that hydraulic entrainment is the dominant pulp loss mechanism. Entrainment was largely affected by flocculation, as rising bubbles tended to be channeled around the flocs. Two regimes of entrainment were proposed, which depended on flocculation. When no flocs were present, the consistency of the foam and feed were similar. In flocculated pulp suspensions, the consistency of the foam represented that of the pulp existing in regions between the flocs. A three-parameter model was derived which predicted the consistencies and the pulp fines fractions of the foams from the flotation experiments. By combining these experimental results with independently-measured pulp sedimentation curves, the model characterized the pulp suspensions' state of flocculation in terms of floc volume fraction, floc consistency and the distribution of pulp fines between the flocculated and non-flocculated regions. The model was more applicable when flow conditions were such that small bubbles were channeled around the flocs. The model also indicated that pulp fines tend to be excluded from flocs.