We　report　on　a　new　kind　of　electrochemical　immunosensors　for　simultaneous　determination　of　the　biomarkers　carcinoembryonic　antigen　(CEA)　and　alpha-fetoprotein　(AFP).　Thionine　and　ferrocene　were　applied　as　distinguishable　electrochemical　tags　(and　mediators)　which　were　covalently　conjugated　on　anti-AFP　and　anti-CEA　antibodies,　respectively,　via　carboxy　groups.　The　resulting　conjugates　were　co-immobilized　on　a　glassy　carbon　electrode　functionalized　with　gold　nanoparticles.　Finally,　horseradish　peroxidase　(HRP)　was　immobilized　onto　the　modified　electrode.　Labeled　thionine　and　ferrocene,　respectively,　act　as　distinguishable　tags　for　simultaneous　determination　of　AFP　and　CEA　due　to　the　difference　in　the　location　of　their　voltammetric　peaks.　With　a　one-step　immunoassay　format,　the　analytes　in　the　sample　produced　transparent　immunoaffinity　reaction　with　the　corresponding　antibodies　on　the　electrode.　Once　the　immunocomplex　is　formed,　it　partially　inhibits　the　active　center　of　the　immobilized　HRP,　and　this　decreased　the　activity　of　HRP　in　terms　of　reduction　of　hydrogen　peroxide.　This　immunosensor　enables　the　simultaneous　determination　of　AFP　and　CEA　in　a　single　run　and　within　the　same　dynamic　range　(0.01-50　ng　mL(-1))　and　the　same　lower　detection　limit　(0.01　ng　mL(-1)).　The　reproducibility　and　stability　of　the　immunosensors　are　acceptable.　The　dual　immunosensor　was　applied　to　evaluate　several　specimens,　and　the　assay　results　are　in　acceptable　agreement　with　clinical　data.