The relation between Eh and electron activity a 3 -commonly found in the literature, namely FEh/2.303RT = pe = -log a e -, is deficient in that the physicochemical state of the electron is not specified. Relations between Eh and a e - (sub (SE)) (the activity of Pt sample electrode electrons) and also between Eh and a e - (sub (aq)) (the activity of aqueous electrons in the sample solution) are derived from a detailed consideration of chemical and electrochemical equilibria in the measurement cell. These relations show that pe SE = -log a e - SE) congruent to O independent of FEh/2.303RT, and that pe aq = log a e - (sub (aq)) = (FEh/2.303RT) + 48.6. It is concluded that FEh/2.303RT, pe SE , and pe aq are three distinctly different parameters and that for practical purposes aqueous electrons do not exist in natural waters.--Modified journal