Provided herein is technology relating to sequencing nucleic acids and particularly, but not exclusively, to devices, methods, and systems for sequencing-by-synthesis using changes in pH to monitor base addition. In some embodiments the electrochemical hydrogen ion sensor is a microfabricated mixed metal oxide electrode and in some embodiments the electrochemical hydrogen ion sensor is a membrane electrode. Moreover, in some embodiments the device further comprises a reference electrode. Performing the sequencing reaction involves moving solutions and other fluids (e.g., samples, nucleotide solutions, wash solutions) into and out of the reaction vessel. Thus, in some embodiments, the device further comprises a tube or other transport mechanism or pathway attached to the reaction vessel.