Prostate cancer (PCa) is the most common cause of mortality in men with cancerous diseases in Western countries. For diagnostic purposes the prostate specific antigen (PSA) is usually used as a sensitive serum marker, but its specificity is limited by a high frequency of falsely elevated values in men with benign prostatic hyperplasia (BPH). Therefore, attention is paid to novel noninvasive biomarkers, which can be microRNA (miRNA). To detect and exactly determine miRNA in samples of healthy and unhealthy persons, Northern blotting, real time RT-PCR, ISH (in situ hybridization), and micro-RNA arrays were usually applied. These methods give especially information on the primary structure level of an miRNA. For the information on the secondary structure level we need further approaches such as application of electrochemical and spectral methods. In addition to the classical characterization of miRNA, represented by PCR, we studied three chosen RNA molecules (miR-23a - chromosome 19p13.13, miR-34a-5p - chromosome 1p36, miR-320a - chromosome 8p21.3) by voltammetry, circular dichroism and UV-Vis spectra. Voltammetric experiments, carried out on carbon and mercury electrodes, indicated different structures of miRNAs depending on pH and other experimental conditions. Structural diversity, conditioning formation i-motifs and multiplexes, was confirmed by CD spectra. The stability of hairpin structures was studied using CD and UV-Vis spectra from which melting points were calculated. MiRNA-320a with the highest content of guanine provided the highest melting point (53 oC at pH 7). With respect to the rapid progress of miRNA electrochemical sensors, our results can be desirable for the research and development of sensitive, selective, robust, portable and time-efficient miRNA sensors which will be able to diagnose cancer and other diseases.