The interaction between long-flame coal particles and oily bubbles controls the crucial processes of attachment and mineralization in oily-bubble flotation thus dominates the recovery of coal. In this study, the interaction energy between long-flame coal particles and diesel oily bubbles was estimated by both DLVO and EDLVO theories, while the attachment of coal particles to oily bubbles was evaluated by induction time measurements and oily-bubble flotation tests. The results indicated that the isoelectric points of oily bubbles and coal particles were observed at about pH 3.5 and 2.3, respectively. Over the entire range of pH 2-11, the attachment between coal particles and oily bubbles was confirmed by the results of the induction time and oily-bubbles flotation tests. Additionally, it was found that the flotation recovery of coals was inversely related to the induction time. The attachment could be accurately predicted by the EDLVO theory over the tested pH range. On the contrary, it showed a great deviation for the DLVO theory to predict the attachment over the tested pH range except at pH 3. The reason for the deviation in prediction may be that the hydrophobic force on the interface of coal particles and oily bubbles is too great to be ignored.