In their article, Gotz et al. (2010) present a categorization methodology for aquatic microcontaminants based on physical–chemical properties and information about degradation and input dynamics. The candidate list in Gotz et al. (2010) contains 250 substances from various compound classes such as biocides, pesticides, human and veterinary pharmaceuticals, estrogens and phytoestrogens, personal care products, mycotoxins, industrial chemicals, and metabolites. In Table S2 of their supporting information, the authors list the candidate substances and corresponding pKa values. The following compounds (and their associated experimental or estimated pKa values; CAS RN are provided in brackets) are not listed as having a pKa by Gotz et al. (2010) (thereby implying they are neutral compounds in all possible aqueous solutions), yet they are clearly ionizable at the pH of the model lake (pH=7) under study: 2-aminobenzimidazole, 7.5 [93432-7] (Sorensen 1999); 2-hydroxy-4-methoxybenzophenone3-(2,2-dichlorvinyl)-2,2-dimethylcyclopropanecarboxylic acid, 4.40 [55701-05-8] (SPARC 2009); 3,5-dibromo-4hydroxybenzoic acid, 3.28 [3337-62-0] (SPARC 2009); 5methylbenzotriazol, 8.85 [136-85-6] (SPARC 2009); amoxicillin, 2.8 and 7.2 [26787-78-0] (Urban et al. 2007); asulam, 4.82 [3337-71-1] (Montgomery 2010); bezafibrat, 3.29 [41859-67-0] (Frimmel and Muller 2006); chlorothalonil-4hydroxy: 1.70 [28343-61-5] (SPARC 2009); clindamycin, 7.6 [18323-44-9] (Porubcan et al. 1978); clofibric acid, 3.18 [88209-7] (Packer et al. 2003); clotrimazole, 6.12 [23593-75-1] (OSPAR 2005); enrofloxacin, 6.0 and 8.8 [93106-60-6] (Riviere and Papich 2009); fenofibric acid, 4.90 [42017-890] (SPARC 2009); fenpropimorph, 6.98 [67306-03-0] (Leistra et al. 2005); fluroxypyr, 2.94 [69377-81-7] (European Food Safety Authority 2011); furosemide, 3.9 [54-31-9] (Manderscheid and Eichinger 2003); ioxitalamic acid, 4.0 [28179-44-4] (SPARC 2009); N4-acetyl-sulfadiazine, 6.1 [127-74-2] (Brittain 2007); N4-acetyl-sulfamethazine, 7.1 [100-90-3] (Fan et al. 2011); norfloxacin, 6.2 and 8.5 [70458-96-7] (Takacs-Novac et al. 1990); ofloxacin, 6.05 and 8.22 [83380-47-6] (Fabre et al. 1994; Barbosa et al. 1998); pyrantel, 12.22 [15686-83-6] (SPARC 2009); sulfadimethoxine, 6.32 [122-11-2] (Brittain 2007); triclosan, 7.9 [3380-34-5] (USFDA/DHHS 2008); venlafaxine, 10.09 [93413-69-5] (Moffat et al. 2004); and zearalenon:, 7.55 [17924-92-4] (SPARC 2009). Furthermore, 4-chlorobenzoic acid (CAS 74-11-3) is incorrectly named as 4-chlor-2-methylphenol. n-Perfluorooctane sulfonic acid is quoted as having a pKa of +4.0, in contrast to its likely value of −5 to −6 (Rayne et al. 2009). Benzyldimethyldodecylammonium chloride (BAC-C12) (CAS 139-07-1), benzyldimethylstearylammonium chloride (BACC18) (CAS 122-19-0), cetalkonium chloride (BAC-C16) (122-18-9), didecyldimethylammonium chloride (DDACC10) (7173-51-5), fluoxetine HCl (Prozac) (CAS 54910-893), and miristalkonium chloride (BAC-C14) (CAS 139-08-2) all appear to be salts whose ionization state/speciation would change upon dissolution. Did the authors attempt to model the aquatic speciation of the less environmentally relevant salt or of the more relevant species that would be formed upon dissolution in water at pH 7? The authors also present the following formulas to calculate the pH-dependent organic carbon–water partitioning coefficient: DOC=(1−a)×KOW×0.41 and a=10/(10+ 10). This simplistic formula incorrectly assumes the Responsible editor: Walter Giger