Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Statement of problem. Coronal and radicular dentin may react differently to the type of acid treatment used when bonding to these substrates. Purpose. The purpose of this study was to characterize and compare dentin morphology, tubular density, and cross-sectional area in deep coronal (around an exposed pulp horn) and root canal dentin at the cervical third in the intact state and after acid etching with phosphoric acid or self-etching dentin primer. Material and methods. Extracted, human, caries-free premolars were fixed. Smear layer-free sections (control) were obtained and divided into 3 groups: left intact, etched with phosphoric acid, or etched with a self-etching primer from a dentin bonding system. Specimens were examined using scanning electron microscopy. Tubular density and cross-sectional area were determined from the images using software. Values were submitted to multifactorial ANOVA (alpha=.05). Results. Tubular density was not significantly affected by acid type or by dentin location. Acid type significantly (P.001) affected tubular cross-sectional area: phosphoric acid > self-etching resin > intact dentin, irrespective of dentin location. All acid-treated specimens showed demineralization, and irregular surface morphology was not affected by either acid treatment. Conclusions. Tubular density was not significantly different between deep coronal and root canal dentin nor affected by acid treatment type. Tubular cross-sectional area did not differ between deep coronal or root canal dentin, regardless of acid treatment. Both acid types significantly increased tubular cross-sectional area at both dentin locations, phosphoric acid to a greater degree than the self-etching primer. Root canal dentin did not display peritubular dentin. (J Prosthet Dent 2010;103:352-361)1036352361Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [05/53290-0]CNPq [310845/2006-8]