The sustainable step-growth polymerization of glycerol is monitored in-line and off-line using an ATR-FTIR spectroscopic probe. The concentration of hydroxyl groups is monitored to provide real-time conversion data using a univariate method. Traditional off-line techniques, i.e., hydroxyl value calculation and water production monitoring, are compared to in-line and off-line ATR-FTIR spectroscopy. Fouling of the probe is observed beyond 42 mol% conversion of hydroxyl groups. No statistically significant differences are found between the conversion data from off-line/in-line ATR-FTIR and two other methods at a 95% confidence level prior to fouling, and this confirms that ATR-FTIR is a reliable tool for monitoring conversion for production of oligoglycerols from a renewable feedstock.