Janus particles are objects with two distinct domains of different surface chemistries that give them an amphiphilic character. The coexistence of hydrophilic and hydrophobic domains makes them suitable models for biological objects (e.g., proteins) and enables their use as "particle surfactants". Since the time of their initial discovery, numerous strategies have been proposed for the realisation of Janus particles with variable and controlled size/functionality of the two surface domains. Owing to experimental limitations though, it has been rarely possible to ascertain and characterise the amphiphilic nature of the particles. We present here new results on the synthesis of spherical Janus microparticles with controlled surface chemistries and Janus ratios and on the measurement of their contact angle in situ at an oil-water interface, using freeze-fracture, shadow-casting cryo-scanning electron microscopy.