Measuring the size of the informal diesel market is important for determining tax revenue losses and identifying inefficiencies in tax policies. The conventional ways of assessing the informal sector entail either not allowing the measurement of the size of the informal sector across time or assuming a stable relationship between diesel consumption and a set of economic variables. This study assesses the informal sector of the Turkish diesel market by using the Kalman filter method. This method allows unobserved values to be estimated with observed variables. Using monthly interpolated GDP, official diesel consumption, and the number of diesel motor vehicles, Turkey's unobserved informal diesel fuel consumption between January 2005 and February 2020 is estimated. The results obtained with this estimation method reveal that the level of informal diesel consumption increased until 2012-2014; it then started to decline at the end of 2014 and started to increase again after 2018. These dates are associated with periods of economic recession, political developments, and the passing of antismuggling legislation.