A hydroprocessing co-catalyst composition may comprise in an embodiment a first component comprising co-catalyst particles and a liquid carrier, and a second component comprising a dispersant and a dispersant diluent. The co-catalyst particles may be in the micron size range, and the dispersant may promote dispersion of the co-catalyst particles in materials such as the liquid carrier, the dispersant diluent, and combinations thereof. Methods of introducing a hydroprocessing co-catalyst composition into a hydroprocessing system are also disclosed.