We are reporting a study of the fundamental and application aspects of positive-ion electrospray ionization (ESI+) for petroleum characterization. In the positive-ion mode, basic nitrogen molecules (typically pyridinic molecules) form predominantly protonated molecule ions. Most other molecular types, such as paraffins, naphthenes, aromatics, and thiophenoaromatics, are largely not ionized by ESI. Metalloporphyrins and neutral nitrogens show little response. The effects of acid and analyte concentrations, and instrumental conditions, were evaluated. Increasing the acid concentration promotes the ionization of the low-molecular-weight bases but has minimal impact on high-molecular-weight species. Analyses of low-and high-molecular-weight petroleum fractions and their mixtures showed minimal mass discrimination. Cone and extraction voltages have a significant impact on ESI responses, particularly on aliphatic amines. Linear dynamic range exists at lower analyte concentrations (<10 -5 M).