In developing countries, the fast spread-out of mobile phone in rural areas has paved the way for many innovations. Since the early 2000's, most Market Information Systems (MIS) have integrated mobile phone, aiming to improve market information access to small farmers, and thereby to ease their insertion into markets and to reduce their asymmetric position in the face of buyers. Most recent studies on MIS are focused on impact assessments, using experimental or quasi-experimental methods to analyze the quantitative impact on prices received by farmers. This communication takes a different perspective. It seeks to understand why the use of these MIS remain marginal and to characterize the users and their access constraints. The method includes two steps: an analysis of the flow of information received and transmitted by the MIS server and a light users' survey. It is applied to two case studies, in Burkina Faso and Tanzania. The results show that the use of these MIS is highly seasonal and concentrated on a few major staple products, despite the wide range of products covered. In addition, most users have only used them on an occasional basis, while regular users are very few. Regular users often turn out to be outreach agents, leaders of farmers' organizations, local officials, rather than small producers. The challenges to manipulate correctly mobile applications for people with very low level of education appear to be a significant obstacle. Beyond the promotion of the tool, the organization of training is a driving force. Targeting intermediate agents, likely to assimilate quickly the use of these tools and to accompany the learning process of smaller famers should be more efficient than targeting directly these latter.