In this paper, we consider learning dictionary models over a network of agents, where each agent is only in charge of a portion of the dictionary elements. This formulation is relevant in big data scenarios where multiple large dictionary models may be spread over different spatial locations and it is not feasible to aggregate all dictionaries in one location due to communication and privacy considerations. We first show that the dual function of the inference problem is an aggregation of individual cost functions associated with different agents, which can then be minimized efficiently by means of diffusion strategies. The collaborative inference step generates local error measures that are used by the agents to update their dictionaries without the need to share these dictionaries or even the coefficient models for the training data. This is a useful property that leads to an efficient distributed procedure for learning dictionaries over large networks.