Cuprous oxide (Cu 2 O)(111) crystals have been prepared at 65°C in weak acidic media (pH 4.7) by electrodeposition. The morphology was observed by scanning electron microscopy and X-ray diffraction analysis was used for investigation of the crystal phase. When a low cathodic current, -0.5 mA/cm 2 , was applied crystals formed on platinum electrode having only one phase, Cu 2 O (111), and cuprous oxide was epitaxially grown with a triangular structure of (111) phase although the electrolysis time was different. The role of a local enhancement of pH at the cathode is observed to have facilitated the deposition of Cu 2 O in weak acidic solution.Recently, various metal oxides such as AgO, 1 Tl 2 O 3 , 2 Cu 2 O, 3,4 and ZnO 5-7 have been prepared by electrochemical deposition due to the following several advantages: (i) thickness and morphology of films can be controlled precisely by adjusting electrochemical parameters, (ii) relatively uniform films can be formed on substrates of complex shapes, (iii) higher deposition rates can be easily obtained, and (iv) the equipment required is inexpensive due to the non requirement of either high vacuum or high reaction temperatures. 8 Cu 2 O is a p-type semiconductor having 2.0 eV bandgap energy and is an attractive photocatalytic material for water splitting into H 2 and O 2 under visible light irradiation. 9 Cuprous oxide is also used as a solar cell and piezoelectric and thermoelectric sensor material. 10 Cu 2 O has been manufactured by electrochemical techniques such as anodic oxidation 11-13 and cathodic deposition. 3,4 In anodic oxidation, under optimal preparation condition, passivating oxide (Cu 2 O) layers are formed on copper in alkaline media and render the metal resistant against corrosion and also show interesting semiconduction properties. Switzer et al. 3 investigated electrodeposition of Cu 2 O in alkaline copper(II) lactate solution and discussed the effect of pH and solution temperature on the film formation. They observed that Cu 2 O crystals have different orientation depending on the pH value of the solution. Cu 2 O films are deposited with (100) four-sided pyramid at pH 9, while Cu 2 O (111) has a triangular structure at pH 12. With increasing temperature, grain size of the films is reported to increase. They used the alkaline solution to make the Cu 2 O film because Cu + is stable in the alkaline solution from Pourbaix diagram. Tanaka et al. 4 investigated the effect of anions on the electrodeposition of Cu 2 O and observed the following during the formation of Cu 2 O and Cu with different cathodic potentials: (i) Cu 2 O has hexahedral, polyhedral crystals and Y-shaped particles in perchlorate, sulfate, and chlorate solutions, respectively, (ii) Cu 2 O was formed and grew larger at higher than -0.55 V (vs. saturated calomel electrode, SCE), while below -0.55 V both Cu and Cu 2 O were deposited on the surface.In this work, we report the observation that cuprous oxide can be prepared in solution with low pH. In acidic solution, Cu + is reduced to Cu metal...