This paper aims to analyze dimensionless parameters, Weber number and Reynolds number, to describe inception of droplets at the interface of liquid–liquid pipe flow. Experiments are conducted in a 15.9 mm diameter and 6 m long stainless-steel pipe using high viscous oils and water. An empirical Weber number correlation, adapted from gas–liquid flow and modified for liquid–liquid flow, is proposed to identify the formation of droplets at the interface. The analysis of the proposed Weber number correlation indicates that the droplets originate when Weber number ≈ 300 and higher. The experimental data from the literature of low and high viscous oil–water flows is also considered for the analysis. The analysis suggests that the Kelvin–Helmholtz instability singularly does not produce droplets at the interface but only when the fluids inertial forces exceed the interfacial tension forces by 300 times. The data from the literature provide sufficient evidence on the existence of wall originated fluctuations in at least one of the fluid phases before the advent of interfacial waves or interfacial droplets. Accordingly, the investigation of Reynolds number indicates that the droplets are found to form when either oil or water phase Reynolds number Reo or Rew attains ≈ 12,000 with the respective fluid mass fraction ≥ 0.2. However, for fluid mass fraction < 0.2, the droplets evolve at the interface when fluid Reynolds number Reo or Rew attains ≈ 3000. © 2021, The Author(s), under exclusive licence to Springer Nature B.V. part of Springer Nature.