In situ ATR-FTIR spectroscopy coupled with in situ image acquisition measurements were used to determine the solubility curve and to investigate the metastable zone width (MSZVV) of ammonium oxalate monohydrate (AO) aqueous solutions under polythermal conditions. The experimental data allowed estimating the MSZW and induction time of the system and the results were compared with published literature values for the same system [1]. "Pseudo" induction time estimation techniques based on polythermal methods showed that AO aqueous solutions exhibit two nucleation regimes depending on the cooling rates. Even though they are based on rough and questionable assumptions, induction time and MSZW estimation methods are often considered as essential for the development of industrial crystallization processes. However, both the relevancy and the physical meaning of the results provided by these methods are uncertain. In the actual industrial context where many advanced measurement techniques and modeling tools became available, the present paper intends to call into question the outcome of the notions of MSZW and induction time. (c) 2011 Elsevier B.V. All rights reserved.