Yacora on the Web (www.yacora.de ) is a web application providing access to collisional radiative models based on the flexible solver Yacora. The main application range is plasma diagnostics in low-pressure plasmas. Available online are three collisional radiative models, namely for atomic hydrogen, molecular hydrogen and helium. This paper gives a brief overview to collisional radiative modeling and to the Yacora solver. The functionality of Yacora on the Web is introduced and the three available models as well as the used input data are presented and discussed. As application example, the models for atomic and molecular hydrogen are applied for investigating spontaneous emission in ionizing and recombining plasmas. This application shows a very good agreement between measured and calculated emission intensities as well as between plasma parameters from other diagnostics and those derived using Yacora on the Web from optical emission spectroscopy results. (C) 2019 Elsevier Ltd. All rights reserved.