The electrocrystallization of Co and Co-Al alloys has been investigated for the first time from a chloroaluminate ionic liquid, AlCl3/[BMIm]Cl-+(-), on Au(111) by in situ electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS). In contrast to Ni deposition (Zell, C. A.; Freyland, W. Proc. Electrochem. Soc. 2002, 19, 660), the formation of a coherent Co monolayer is not observed in the underpotential range. Instead, 2D phase formation sets in at slightly cathodic potentials characterized by monatomically high Co islands with a radius of 2-3 nm and a narrow size distribution. Their growth behavior has been studied as a function of time and overpotential eta in the range -0.05 less than or equal to eta/V less than or equal to -0.17 versus Co/Co(II). It can be described by a classical nucleation model yielding a critical nucleus of 1-2 atoms. At potentials below -0.2 V, a fast 3D growth of Co clusters is manifest. Although Co and An are immiscible in the bulk phase at room temperature, the,STM images give clear evidence of surface alloying for 2D and 3D cluster deposits. Co-deposition of Co and Al has been studied in the potential range -0.3 less than or equal to eta/V less than or equal to -0.7. It is found that the grain size of the CoxAl1-x clusters decreases with increasing Al content. Particularly remarkable is the result that the composition of CoxAl1-x can be controlled in situ with nanometer resolution on a time scale of milliseconds, the typical time for recording the tunneling spectra being similar to200 mus. This is demonstrated by the variation of the effective tunneling barrier phi as a function of eta as determined from STS measurements. It quantitatively agrees with independent composition determinations by conventional electrochemical methods. This correlation has not been reported before.