In this work, the formation of clusters in a supersonic jet expansion has been investigated. Clusters of argon, carbon-dioxide, nitrogen, and oxygen have been generated in the gas jet with three different types of nozzles (0.5 mm diameter, 0.8 mm diameter, and a conical nozzle with 0.8 mm diameter aperture). The mean size and density of the clusters have been measured as a function of backing pressure. Here, density of clusters refers to the density of the cluster collection. Rayleigh scattering experimental data and Mach-Zehnder interferometer data have been combined to estimate the average size and density of clusters. CO₂ and Ar clusters form easily at about a backing pressure of 2 bars, while N₂ and O₂ clusters do not form easily. N₂ and O₂ begin to cluster at a backing pressure of about 18–24 bars. The mean cluster radius of Ar varies from 6.6 A ̊ to 24 Å and the cluster radius of CO₂ varies from about 8.8 A ̊to 35 Å as the backing pressure increases from 3 bars to 14 bars. In the same range of backing pressures, the average cluster gas density changes from 37.3 X10¹⁵cm^-3 to 1.6X10¹⁵cm^-3 for Ar clusters and changes from 10.52 X10¹⁵cm^-3 to 0.22X 10¹⁵cm^-3 for CO₂ clusters. It is assumed that the condensation into clusters is total. The mean cluster radius of N₂ is smaller and varies approximately from 4.2 Å to 8.6 Å and the cluster radius of O₂ varies from about 4.9 Å to 12.4 Å as the backing pressure increases in the range of 18 bars to 35 bars. In the same range of backing pressures, the average cluster density changes from 48.2X10¹⁶cm^-3 to 3.7X10¹⁶cm^-3 for N₂ clusters and from 21.5X10¹⁶cm^-3 to 1.3X10¹⁶cm^-3 for O₂ clusters.