This paper investigates personal knowledge development in online learning environments using the perspective of a model adapted from Nonaka and colleagues’ SECI model. To this end, the SECI model, which was originally designed to describe organisational knowledge creation and conversion, was adapted to conceptualise personal knowledge development in online learning at the individual level. As the SECI model was originally conceived at the organisational level, in order to measure personal knowledge development at the individual level in the context of online learning, a measurement instrument was created in order to measure the scores of individual online learners on Externalisation, Combination and Internalisation. It is argued that Socialisation is not a relevant mode in the context of online learning and is therefore not covered in the measurement instrument; this is explained further in the paper. This measurement instrument also examines the interrelationships between the three modes and a new model – the so‑called EC‑I model – is proposed to depict these interrelationships. The measurement instrument is based on data collected through an online survey, in which online learners report on their experiences of personal knowledge development in online learning environments. In other words, the instrument measures the magnitude of online learners’ Externalisation and Combination activities as well as their level of Internalisation, i.e. the outcomes of their personal knowledge development in online learning. For Externalisation and Combination, formative indicators were used, whereas for Internalisation reflective indicators were used. The measurement instrument is one of the main foci of this paper and is therefore discussed in‑depth. In sum, the paper proposes a modified version of the SECI model, extending the applicability of the original SECI model from the organisational to the individual level. It outlines a new measurement instrument which can be used to measure Externalisation and Combination, i.e. personal knowledge development processes, and Internalisation, i.e. personal knowledge development outcomes.