The formation mechanism of brown dwarfs (BDs) and its relation to star and planet formation remains an open issue. A careful determination of the substellar mass function in open clusters at different ages and environments is one of the primary methods for addressing this. We have performed a deep, wide area, multiband photometric survey from which we have identified low mass stellar and BD candidates in the open cluster IC 2391 (age~50 Myr, D~146 pc). Here we will use multi-object spectroscopy to determine their spectral type, H(alpha) emission, Li absorption and radial velocity from which we can confirm or refute cluster membership case-by-case and estimate their (model-based) masses. From this we will determine the substellar mass function to much lower mass objects than has hitherto been performed in a cluster of this age. Comparing this with the MF is other clusters at different ages we will learn about the evolution of the substellar population.