The substitution of carbonyl ligands by isonitriles on Co₄(μ₄-SiC₆H₄R)₂(CO)₁₁ (R=H, OMe, NMe₂) clusters has been examined. The reactions could be monitored by electrospray ionisation mass spectrometry, identifying the substituted products as either protonated [M+H]⁺ (R=NMe₂) or oxidised [M]⁺ (R=H, OMe) ions. Evidence for substitution of up to nine carbonyl ligands was found and the structure of tetra-substituted example, Co₄(μ₄-SiC₆H₄OMe)₂(CO)₇(XyNC)₄, was determined.