The structure of mixed acid / decyl monolayers (MLs) grafted on oxide free Si(111) surfaces by photochemical hydrosilylation in a mixture of neat undecylenic acid and 1-decene is studied in detail. After appropriate surface cleaning of as grafted surfaces, AFM (topography and phase imaging) and calibrated FTIR analysis demonstrate that a mixed monolayer is formed, free of residue. When the acid-molecule fraction (Γ SOL) is > 0.1 mixed monolayers are homogeneous on the scale of observations and they are only slightly enriched in acid chains with respect to the solution. Conversely, when Γ SOL < 0.1 the acid chain fraction within the ML becomes quasiindependent of the solution composition and may become much larger than Γ SOL. In addition, dark domains are observed in AFM phase images. Correlations between the characteristic parameters of CO IR band and AFM phase images suggest a strong phase separation of acid and alkyl chains at various length scales. A model involving a structuration of the grafting solution is proposed to explain observations.